Magnetic recording medium

ABSTRACT

A magnetic recording medium which comprises a support having thereon a substantially nonmagnetic lower layer and a magnetic layer comprising a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder dispersed in a binder provided on the lower layer, which is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch 2  of areal recording density, wherein the coercive force of the magnetic layer is 1,800 Oe or more, and the thickness unevenness of the support is 5% or less of the thickness of the support.

FIELD OF THE INVENTION

The present invention relates to a coating type high capacity magneticrecording medium capable of high density recording. More specifically,the present invention relates to a coating type high capacity magneticrecording medium for high density recording which comprises a magneticlayer on a substantially nonmagnetic lower layer wherein the uppermostmagnetic layer contains a ferromagnetic metal powder or a hexagonalferrite powder.

BACKGROUND OF THE INVENTION

In the field of a magnetic disc, a 2 MB MF-2HD floppy disc usingCo-modified iron oxide has been generally loaded in a personal computer.However, along with the increase in the amount of data to be dealt with,the capacity thereof has become insufficient and the increase of thecapacity of the floppy disc has been demanded.

Also, in the field of a magnetic tape, with the prevalence of an officecomputer, such as a minicomputer, a personal computer and a workstation, a magnetic tape for recording computer data as external storagemedium (a so-called backup tape) has been vigorously studied. For therealization of the magnetic tape for such a use, the improvement ofrecording capacity has been strongly demanded conjointly with theminiaturization of a computer and the increase of information processingability (e.g., information throughput).

Magnetic layers comprising an iron oxide, a Co-modified iron oxide,CrO₂, a ferromagnetic metal powder, or a hexagonal ferrite powderdispersed in a binder, which are coated on a nonmagnetic support, havebeen conventionally widely used in magnetic recording media.Ferromagnetic metal powders and hexagonal ferrite powders among thesehave been known to have excellent high density recordingcharacteristics.

In the case of a disc, as high capacity discs using ferromagnetic metalpowders which are excellent in high density recording characteristics,there are 10 MB MF-2TD and 21 MB MF-2SD, and as high capacity discsusing hexagonal ferrite, there are 4 MB MF-2ED and 21 MB Floptical,however, any of these are not satisfactory with respect to capacitiesand properties. As is the circumstance, various attempts have been madeto improve high density recording characteristics. Some examples thereofare described below.

For improving characteristics of a disc-like magnetic recording medium,JP-A-64-84418 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”) proposes the use of a vinylchloride resin having an acidic group, an epoxy group and a hydroxylgroup, JP-B-3-12374 (the term “JP-B” as used herein means an “examinedJapanese patent publication”) proposes the use of a metal powder havinga coercive force Hc of 1,000 Oe or more and a specific surface area offrom 25 to 70 m²/g, and JP-B-6-28106 proposes to regulate the specificsurface area and magnetic susceptibility of magnetic powders and containan abrasive.

For improving the durability of a disc-like magnetic recording medium,JP-B-7-85304 proposes the use of a fatty acid ester having anunsaturated fatty acid ester and an ether bond, JP-B-7-70045 proposesthe use of a fatty acid ester having a branched fatty acid ester and anether bond, JP-A-54-124716 proposes the use of a nonmagnetic powderhaving a Mohs' hardness of 6 or more and a higher fatty acid ester,JP-B-7-89407 proposes to regulate the volume of voids containing alubricant and the surface roughness to from 0.005 to 0.025 μm,JP-A-61-294637 proposes the use of a fatty acid ester having a lowmelting point and a fatty acid ester having a high melting point,JP-B-7-36216 proposes the use an abrasive having a particle size of from¼ to ¾ of the magnetic layer thickness and a fatty acid ester having alow melting point, and JP-A-3-203018 proposes the use of a metallicmagnetic powder containing Al and a chromium oxide.

As the constitution of a disc-like magnetic recording medium having anonmagnetic lower layer and an intermediate layer, JP-A-3-120613proposes the constitution comprising an electrically conductive layerand a magnetic layer containing a metal powder, JP-A-6-290446 proposesthe constitution comprising a magnetic layer having a thickness of 1 μmor less and a nonmagnetic layer, JP-A-62-159337 proposes theconstitution comprising an intermediate layer comprising a carbon and amagnetic layer containing a lubricant, and JP-A-5-290358 proposes theconstitution comprising a nonmagnetic layer in which the carbon size isregulated.

On the other hand, a disc-like magnetic recording medium comprising athin magnetic layer and a functional nonmagnetic layer has beendeveloped in recent years and floppy discs of the class with thecapacity of 100 MB are now on the market. As floppy discs showing thesecharacteristics, JP-A-5-109061 proposes the constitution comprising amagnetic layer having Hc of 1,400 Oe or more and a thickness of 0.5 μmor less and a nonmagnetic layer containing electrically conductiveparticles, JP-A-5-197946 proposes the constitution comprising abrasiveshaving particle sizes larger than the thickness of the magnetic layer,JP-A-5-290354 proposes the constitution comprising a magnetic layerhaving the thickness of 0.5 μm or less and the fluctuation of thethickness of within ±15%, in which the surface electric resistance isregulated, and JP-A-6-68453 proposes the constitution in which two kindsof abrasives having different particle sizes are contained and theamount of the abrasives on the surface is regulated.

Further, in the field of a tape-like magnetic recording medium, with theprevalence of an office computer, such as a minicomputer and a personalcomputer, a magnetic tape for recording computer data as externalstorage medium (a so-called backup tape) has been vigorously studied.For the realization of the magnetic tape for such use, the improvementof recording capacity has been strongly demanded conjointly with theminiaturization of a computer and the increase of information processingcapability. In addition, the use in various environmental conditions dueto widening of use environments of magnetic tapes (in particular, underfluctuating temperature/humidity conditions), reliability on datastorage, and reliability on performance, such as stablerecording/readout of data in multiple running due to repeating use athigh speed, have been increasingly demanded.

Magnetic tapes which are used in digital signal recording systems varyaccording to each system, for example, magnetic tapes corresponding to aso-called DLT type, 3480, 3490, 3590, QIC, a D8 type and a DDS type areknown. In every system, the magnetic tape comprises, on one surface sideof a nonmagnetic support, a magnetic layer of a single layer structurehaving a comparatively thick layer thickness, e.g., from 2.0 to 3.0 μm,containing a ferromagnetic powder, a binder and an abrasive, and a backcoating layer provided on the surface side of the support opposite tothe side having the magnetic layer for purposes of preventing windingdisarrangement and maintaining good running durability. However, ingeneral, in a magnetic layer of a single layer structure having acomparatively thick layer thickness as described above, there is aproblem of thickness loss which generates the reduction of output.

For the improvement of the reduction of reproduction output due tothickness loss, thinning of a magnetic layer has been known. Forexample, JP-A-5-182178 discloses a magnetic recording medium comprisinga nonmagnetic support having thereon a lower nonmagnetic layercontaining an inorganic powder dispersed in a binder and an uppermagnetic layer containing a ferromagnetic powder dispersed in a binderand having a thickness of 1.0 μm or less, which is coated on the lowernonmagnetic layer while the nonmagnetic layer is still wet.

However, with the rapid trend of the increase of the capacity anddensity of disc-like and tape-like magnetic recording media, it hasbecome difficult to obtain satisfactory characteristics even with thesetechniques. It has also become difficult to make the increase of thecapacity and density compatible with durability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a magnetic recordingmedium which is markedly improved in electromagnetic characteristics, inparticular, high density recording characteristics, and which uniteshigh durability, in particular, an error rate in high density recordingregion is conspicuously improved. Specifically, an object of the presentinvention is to provide a high capacity magnetic recording medium, inparticular, a disc-like magnetic recording medium having a highrecording capacity of from 0.17 to 2 G bit, preferably from 0.2 to 2 Gbit, and particularly preferably from 0.35 to 2 G bit.

As a result of earnest studies to provide a high capacity magneticrecording medium which is excellent in electromagnetic characteristicsand durability, in particular, markedly improved in an error rate in ahigh density recording region, the present inventors have found thathigh capacity, high density recording characteristics and excellentdurability of the object of the present invention can be obtained by themagnetic recording medium having the constitution described below, thusthe present invention has been attained.

That is, the present invention can be attained by a magnetic recordingmedium which comprises a support having thereon a substantiallynonmagnetic lower layer and a magnetic layer comprising a ferromagneticmetal powder or a ferromagnetic hexagonal ferrite powder dispersed in abinder provided on the lower layer, which is a magnetic recording mediumfor recording signals of from 0.17 to 2 G bit/inch² of areal recordingdensity, wherein the coercive force of the magnetic layer is 1,800 Oe ormore, and the thickness unevenness of the support is 5% or less of thethickness of the support, preferably the thickness unevenness of thesupport is 2% or less of the thickness of the support, or the presentinvention can be attained by a magnetic recording medium which comprisesa support having thereon a substantially nonmagnetic lower layer and amagnetic layer comprising a ferromagnetic metal powder or aferromagnetic hexagonal ferrite powder dispersed in a binder provided onthe lower layer, which is a magnetic recording medium for recordingsignals of from 0.17 to 2 G bit/inch² of areal recording density,wherein the coercive force of the magnetic layer is 1,800 Oe or more,and the ratio of the spatial frequency intensity of long wavelength ofthe surface roughness of from 10 to 2 μm of the magnetic layer (I_(L))to the spatial frequency intensity of short wavelength of the surfaceroughness of from 1 to 0.5 μm of the magnetic layer (I_(S)),(I_(L)/I_(S)), is less than 1.5. The magnetic layer preferably has a drythickness of from 0.05 to 0.30 μm, more preferably from 0.05 to 0.25 μm,φm of preferably from 10.0×10⁻³ to 1.0×10⁻³ emu/cm², more preferablyfrom 8.0×10⁻³ to 1.0×10⁻³ emu/cm² and the magnetic recording medium ofthe present invention is a magnetic recording medium for recordingsignals of preferably from 0.17 to 2 G bit/inch² of areal recordingdensity. The present inventors have found that the magnetic recordingmedium having high capacity, excellent high density characteristics andexcellent durability, in which, in particular, the error rate in highdensity recording region has been markedly improved, which could not beobtained by conventional techniques, could be obtained by adopting theconstitution of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The term “a substantially nonmagnetic lower layer” in the presentinvention means a lower layer which may have a magnetic property but nottoo much for participating in recording and hereinafter referred tosimply as “a lower layer” or “a nonmagnetic layer”. When a magneticpowder is contained in the lower layer, the content is preferably lessthan ½ of the content of the inorganic powder.

Areal recording density is a value obtained by multiplying linearrecording density by track density.

φm is the amount of magnetic moment (emu/cm²) which can be directlymeasured from the magnetic layer per unit area of one side using avibrating sample magnetometer (VSM, a product of Toei Kogyo Co., Ltd.)at Hm 10 KOe, which is equal to the value obtained by multiplyingmagnetic flux density (Bm) obtained using VSM (unit G=4πemu/cm³) by thethickness (cm). Accordingly, the unit of φm is represented by emu/cm² orG•cm. Further, the thickness unevenness of the support is measured asfollows.

The thicknesses of the support in each of the machine direction andtransverse direction are recorded on a recorder using an electronmicrometer. The difference between the altitudes of the highest peak andthe deepest valley in the machine direction and the difference betweenthe altitudes of the highest peak and the deepest valley in thetransverse direction of the designated measurement length (within 2meters) are read out, the larger value of the altitude differences istaken and the value obtained by dividing this altitude difference by thethickness of the support and multiplying 100 is taken as the thicknessunevenness (it is represented by Δt).

The thickness of the support is measured using a micrometer by pilingten sheets of supports and measuring thicknesses at three arbitrarypoints. The average value of three points is found and this averagevalue is divided by 10, which is taken as the thickness of the support.

Linear recording density means the bit number of signals recorded per 1inch in the recording direction.

These linear recording density, track density and areal recordingdensity are values determined by each system.

The spatial frequency intensity in long wavelength of from 10 to 2 μm ofthe surface roughness of the magnetic layer (I_(L)) and the spatialfrequency intensity in short wavelength of from 1 to 0.5 μm of thesurface roughness of the magnetic layer (I_(S)) are intensities obtainedby conducting two dimensional Fourier transformation treatment to thesurface roughness profile data of the magnetic layer, then factorizingthe roughness components of every wavelength, and integrating within therange of the corresponding wavelength components. These are valuescalculated by the uptake of the range of 100 μm×100 μm as data of512×512 pixels using an atomic force microscope (AFM) (manufactured byDigital Instruments, U.S.A.).

That is, the present inventors have elaborated some means in an attemptto improve the magnetic layer thickness, the coercive force (Hc) and thecentral plane average surface roughness as to the linear recordingdensity, and the optimization of the φm as to the track density for theimprovement of the areal recording density. That is, it has beenconventionally known that the use of a smooth support is preferred, butthis has been a countermeasure to cope with a problem such that if thethickness of the support is increased, Δt is also increased, whichhinders smooth coating.

However, the present inventors have found that by making Δt of a support5% or less, preferably 2% or less, head touch can be conspicuouslyimproved, fluctuation of output can be controlled, as a result, theerror rate in high density recording region can be reduced.

A support having Δt of 5% or less, preferably 2% or less, can beproduced as follows. Further, materials of a support are notparticularly restricted but flexible supports mainly comprising resinsare advantageous in the present invention from the productivity and theeconomical standpoint.

The following methods can be exemplified as means of making Δt 5% orless but the methods are not particularly limited thereto.

(1) A method of lessening discharge distribution of a molten polymer(e.g., PET, PEN), a polymer solution (aramide) from the die in thetransverse direction, which contributes to the reduction of thethickness unevenness in the transverse direction.

(2) A method of lessening discharge fluctuation of a molten polymer(e.g., PET, PEN), a polymer solution (aramide) from the die, whichcontributes to the reduction of the thickness unevenness in the machinedirection.

(3) A method of increasing percent of stretching in the machine andtransverse directions.

The ratio of the spatial frequency intensity in long wavelength of from10 to 2 μm of the surface roughness of the magnetic layer (I_(L)) to thespatial frequency intensity in short wavelength of from 1 to 0.5 μm ofthe surface roughness of the magnetic layer (I_(S)), (I_(L)/I_(S)), ofconventional magnetic recording media, in particular, floppy discs, hasbeen usually more than 1.5. This means that the undulation ofconventional magnetic layer surface is far larger than the minuteprotrusion. It can be thought that electromagnetic characteristics anddurabilities of magnetic layers are determined by this undulationcomponent.

However, I_(L)/I_(S) in the present invention is less than 1.5. By theconstruction of the magnetic recording medium of the present inventionwhich comprises a magnetic layer having such surface property, amagnetic recording medium having excellent high density characteristicswhich unites high durability and in which in particular, an error ratein high density recording region has been remarkably improved, can beobtained.

As described later, with respect to the surface roughness defined by Ra,wavelength (which intimately correlates with the distribution ofundulation and protrusion) is not particularly limited and the averagesurface roughness in a specific range is expressed numerically. However,the scope of the present invention clearly defines the wavelength andcontrols the undulation not as a mere numerical value, and aims atensuring running durability by specifying the distribution ofprotrusions. Accordingly, even if Ra values are the same, Ra valuehaving a large undulation component cannot obtain the effect of thepresent invention, and the effect of the present invention can beexhibited by making the undulation component small in a certain degreeto satisfy the above-described I_(L)/I_(S).

As described above, I_(L) and I_(S) are intensities obtained byconducting two dimensional Fourier transformation treatment to thesurface roughness profile data of the magnetic layer, then factorizingthe roughness components of every wavelength, and then integrating therange of the corresponding wavelength component. Accordingly, longwavelength components are largely decreased in the magnetic layer of thepresent invention as compared with conventional magnetic layers andshort wavelength components rather predominate.

In the present invention, as a means for making the surface property ofthe magnetic layer, I_(L)/I_(S), less than 1.5, the use of ahyper-smooth support is particularly effective, which is describedlater. In addition to the above, controlling the shapes and sizes ofvarious powders contained in the upper and lower layers, appropriatelyselecting the combination of a coating condition (W/W coating or W/Dcoating) and an orientation condition after coating, and appropriatelyselecting a surface treatment condition such as a calendering treatmentor a burnishing treatment can be exemplified as other means.

In the present invention, the range of I_(L)/I_(S) is preferably0.5≦I_(L)/I_(S)≦1.3, more preferably 0.8≦I_(L)/I_(S)≦1.1.

Preferred embodiments of the present invention are described below.

As for the Magnetic Recording Medium as a Whole:

(1) In the above magnetic recording medium, the magnetic layer has acentral plane average surface roughness of 4.0 nm or less measured by3D-MIRAU method.

(2) In the above magnetic recording medium, the magnetic layer has acoercive force of 2,100 Oe or more, and the ferromagnetic metal powderhas an average long axis length of 0.12 μm or less or the ferromagnetichexagonal ferrite powder has an average particle size of 0.10 μm orless.

(3) The above magnetic recording medium is a magnetic recording mediumfor recording signals of from 0.20 to 2 G bit/inch² of areal recordingdensity.

(4) The above magnetic recording medium is a magnetic recording mediumfor a system of a high transfer rate of 1.0 MB/sec. or more.

(5) The above magnetic recording medium is a magnetic recording mediumfor a high capacity floppy disc system of disc rotation speed of 2,000rpm or more.

As for the Improvement of the Magnetic Powder:

(1) In the above magnetic recording medium, the ferromagnetic metalpowder comprises Fe as a main component, has an average long axis lengthof from 0.12 μm or less, and an acicular ratio of from 4.0 to 9.0.

(2) In the above magnetic recording medium, the ferromagnetic metalpowder comprises Fe as a main component, has an average long axis lengthof 0.10 μm or less, and a crystallite size of from 80 to 180 Å.

As for the Improvement of the Support:

(1) In the above magnetic recording medium, the support has a centralplane average surface roughness of 4.0 nm or less.

(2) In the above magnetic recording medium, the support has a thermalshrinkage factor of 0.5% or less both at 100° C. for 30 minutes and at80° C. for 30 minutes in every direction of in-plane of the support.

(3) In the above magnetic recording medium, the support has atemperature expansion coefficient of from 10⁻⁴ to 10⁻⁸/° C. in everydirection of in-plane of the support.

(4) In the above magnetic recording medium, the support has an F-5 valueof from 49 to 490 MPa.

(5) In the above magnetic recording medium, the support has a breakingstrength of from 49 to 980 MPa.

As for the Improvement of Lubricants:

(1) In the above magnetic recording medium, the lower layer and/or themagnetic layer contain(s) at least three kinds in total of a fatty acidand/or a fatty acid ester.

(2) In the above magnetic recording medium, the fatty acid and the fattyacid ester have the same fatty acid residues with each other.

(3) In the above magnetic recording medium, the fatty acid contains atleast a saturated fatty acid and the fatty acid ester contains at leasta saturated fatty acid ester or an unsaturated fatty acid ester.

(4) In the above magnetic recording medium, the fatty acid estercontains a monoester and a diester.

(5) In the above magnetic recording medium, the fatty acid estercontains a saturated fatty acid ester and an unsaturated fatty acidester.

(6) In the above magnetic recording medium, the surface of the magneticlayer has a C/Fe peak ratio of from 5 to 120 when the surface ismeasured by the Auger electron spectroscopy.

As for the Improvement of the Nonmagnetic Powder for the Lower Layer:

(1) In the above magnetic recording medium, the lower layer contains acarbon black having a particle size of from 5 to 80 mμ and the magneticlayer contains a carbon black having a particle size of from 5 to 300mμ.

(2) In the above magnetic recording medium, the lower layer contains acarbon black having an average particle size of from 5 to 80 mμ and acarbon black having an average particle size of larger than 80 mμ.

(3) In the above magnetic recording medium, the lower layer and themagnetic layer each contains a carbon black having an average particlesize of from 5 to 80 mμ.

(4) In the above magnetic recording medium, the lower layer contains anacicular inorganic powder having an average long axis length of 0.20 μmor less and an acicular ratio of from 4.0 or 9.0.

(5) In the above magnetic recording medium, the lower layer contains anacicular inorganic powder and the magnetic layer contains an acicularferromagnetic metal powder, and the average long axis length of theacicular inorganic powder is from 1.1 to 3.0 times the average long axislength of the acicular ferromagnetic metal powder.

(6) In the above magnetic recording medium, the lower layer and/or themagnetic layer contain(s) a phosphorus compound and the lower layercontains an acicular or spherical inorganic powder.

As for the Improvement of the Abrasive for the Magnetic Layer:

(1) In the above magnetic recording medium, the magnetic layer containsat least an abrasive having an average particle size of from 0.01 to0.30 μm.

(2) In the above magnetic recording medium, the magnetic layer containsat least a diamond particle having an average particle size of from 0.01to 1.0 μm.

(3) In the above magnetic recording medium, the magnetic layer containstwo kinds of abrasives having a Mohs' hardness of 9 or more.

(4) In the above magnetic recording medium, the magnetic layer containsan α-alumina and a diamond particle.

As for the Improvement of the Binder:

(1) In the above magnetic recording medium, the lower layer and/or themagnetic layer contain(s) at least a polyurethane having a glasstransition temperature of from 0° C. to 100° C.

(2) In the above magnetic recording medium, the lower layer and/or themagnetic layer contain(s) at least a polyurethane having a breakingstress of from 0.05 to 10 kg/mm².

As for the Improvement of the Magnetic Recording Medium as a Whole:

(1) In the above magnetic recording medium, the magnetic layer has a drythickness of from 0.05 to 0.20 μm and the magnetic layer contains anabrasive having an average particle size of 0.4 μm or less.

(2) The above magnetic recording medium is a disc-like magneticrecording medium.

(3) The above magnetic recording medium is a magnetic recording mediumfor a high capacity floppy disc system of disc rotation speed of 3,000rpm or more.

(4) The above magnetic recording medium is a magnetic recording mediumfor a system of a high transfer rate of 2.0 MB/sec. or more.

(5) The above magnetic recording medium is a magnetic recording mediumwhich has realized subordination transposition capable ofrecording/reproduction with conventional 3.5 inch type floppy discs.

(6) The above magnetic recording medium is a magnetic recording mediumfor a high capacity floppy disc system adopting a dual discrete gap headhaving both a narrow gap for high density recording and a broad gap forconventional 3.5 inch type floppy discs.

(7) The above magnetic recording medium is a magnetic recording mediumfor a high capacity floppy disc system adopting a head which floats bydisc rotation.

(8) The above magnetic recording medium is a magnetic recording mediumfor a high capacity floppy disc system adopting a head which floats bydisc rotation and, at the same time, a linear type voice coil motor as adriving motor of the head.

The present inventors have found that a magnetic recording medium, inparticular, a disc-like magnetic recording medium, in the recordingcapacity system of areal recording density of from 0.17 to 2 Gbit/inch², having excellent high density characteristics and excellentdurability, in particular, markedly improved error rate in high densityrecording region, which could not be obtained by conventionaltechniques, could be obtained by adopting the above constitution of thepresent invention.

A magnetic recording medium, in particular, a disc-like magneticrecording medium, having high density characteristics and highdurability in the recording capacity system of areal recording densityof from 0.17 to 2 G bit/inch², preferably from 0.2 to 2 G bit/inch²,more preferably from 0.35 to 2 G bit/inch², which could never beachieved by any coating type magnetic recording media known in theworld, can be obtained as a result of organically combining andsynthesizing the points as shown below.

The points aimed at in the present invention include (1) high Hc andhyper-smoothing, (2) ensuring of durability by the improvement of acomposite lubricant, a binder with high durability and a ferromagneticpowder, and the use of an abrasive with high hardness (3) ultra-thinningof the magnetic layer and the reduction of fluctuation in the interfacebetween the magnetic layer and the lower layer, (4) the increase ofpacking density of powders (a ferromagnetic powder and a nonmagneticpowder), (5) ultra-fine granulation of powders (a ferromagnetic powderand a nonmagnetic powder), (6) stabilization of head touch, (7)dimensional stability and servomechanism, (8) improvement of thermalshrinkage factors of the magnetic layer and the support, and (9) thefunctions of a lubricant at high temperature and low temperature, andthe present invention has been achieved by combining and synthesizingthese points.

The above item (1) high Hc and hyper-smoothing are described in thefirst place. Hc of the magnetic layer can be increased to 1,800 Oe ormore, preferably 2,100 Oe or more, by using a ferromagnetic powder withhigh Hc, thereby high capacity and high density can be obtained. Withrespect to hyper-smoothing, a smooth magnetic layer can be obtained bymaking the central plane surface roughness preferably 4.0 nm or less,and employing ATOMM® structure. High capacity and high density can beattained by the central plane surface roughness of preferably 4.0 nm orless. Subsequently, the above item (2) ensuring of durability by theimprovement of a composite lubricant, a binder with high durability anda ferromagnetic powder, and the use of an abrasive with high hardness isdescribed. With respect to a composite lubricant, fundamental conceptsfor the enhancement of lubrication capability are shown below.

(1) A plurality of lubricants having different functions andcapabilities are used in combination.

(2) A plurality of lubricants having similar functions and capabilitiesare used in combination.

Due to the above item (1), a variety of functions and capabilities canbe attained under various conditions. Further, due to the above item(2), affinity and compatibility of lubricants with each other can beensured and good functions of lubricants can be exhibited.

Examples of combinations of a plurality of lubricants having differentfunctions and capabilities as in the above item (1) are shown below.

1) A Lubricant having a fluid lubrication function and a lubricanthaving a boundary lubrication function are used in combination.

2) A polar lubricant and a nonpolar lubricant are used in combination.

3) A liquid lubricant and a solid lubricant are used in combination.

4) Lubricants having different polarities, in particular, a fatty acidand/or a fatty acid ester, are used in combination. For example, a fattyacid monoester and a fatty acid diester are used in combination.

5) Lubricants having different melting points and different boilingpoints, in particular, a fatty acid and/or a fatty acid ester, are usedin combination.

6) Lubricants which differ in lengths of carbon atom number, inparticular, a fatty acid and/or a fatty acid ester, are used incombination.

7) A straight chain lubricant and a branched chain lubricant, inparticular, a fatty acid and/or a fatty acid ester, are used incombination. For example, a straight chain fatty acid ester and abranched fatty acid ester are used in combination.

8) A lubricant having saturated carbon chain and a lubricant havingunsaturated carbon chain, in particular, a fatty acid and/or a fattyacid ester, are used in combination. For example, a saturated fatty acidester and an unsaturated fatty acid ester are used in combination.

9) Lubricants having different affinities with a binder are used incombination.

10) Lubricants having different affinities with an inorganic powder areused in combination.

By the combined use of lubricants as in the above item (1), a variety offunctions and capabilities can be attained under various conditions.

Examples of combinations of a plurality of lubricants having similarfunctions and capabilities as in the above item (2) are shown below.

1) Fatty acid residues of a fatty acid and a fatty acid ester are madethe same with each other.

2) Fatty acid esters having the same fatty acid residues with each otherand/or having the same alcohol residues with each other are used incombination.

3) Two or more saturated fatty acids are used in combination.

4) Saturated fatty acids are respectively used in the fatty acid residueparts of a fatty acid and a fatty acid ester.

5) Unsaturated fatty acids are respectively used in the fatty acidresidue parts of a fatty acid and a fatty acid ester.

6) Three or more fatty acid esters alone are used in combination.

7) Fatty acid parts of a fatty acid and a fatty acid amide are made thesame with each other.

By the combined use of lubricants as in the above item (2), affinity andcompatibility of lubricants with each other can be ensured and goodfunctions of lubricants can be exhibited.

By the combined use of lubricants as in the above items (1) and (2), notonly a variety of functions and capabilities can be attained undervarious conditions but also affinity and compatibility of lubricantswith each other can be ensured and good functions of lubricants can beexhibited.

The binder with high durability is described below. By the incorporationof a resin having a polar group, in particular, a polyurethane resin,having high dispersibility, high glass transition temperature and highbreaking stress, durability of the binder can be improved. It ispreferred for the polyurethane resin to have 2 or more OH groups, morepreferably 3 or more, and most preferably 4 or more, at the terminal ofthe molecule because the reactivity with polyisocyanate, which is apolyfunctional curing agent, becomes high and the coated film of threedimensional network can be formed after curing. With respect to theimprovement of the ferromagnetic powder, durability can be improved byincreasing the Al component which can heighten the hardness of theferromagnetic powder. Ensuring of durability by using an abrasive havinghigh hardness is described below. Higher durability can be obtained bythe combined use of a diamond fine particle having a Mohs' hardness of10 with conventionally used abrasives having a Mohs' hardness of about9, e.g., an α-alumina. Next, ultra-thinning of the magnetic layer andthe reduction of fluctuation in the interface between the magnetid layerand the lower layer in the above item (3) is described. By reducing thethickness of the magnetic layer to preferably from 0.05 to 0.30 μm, morepreferably from 0.05 to 0.25 μm, and reducing the fluctuation in theinterface between the magnetic layer and the lower layer, uniform,smooth and thin magnetic layer can be obtained, thereby higher capacityand higher density of the magnetic recording medium can be attained. Theincrease of the packing density of powders (a ferromagnetic powder and anonmagnetic powder) in the above item (4) is described. By high packingdensity of a fine ferromagnetic powder, specifically, a fineferromagnetic metal powder, preferably having an average particle lengthof 0.15 μm or less, more preferably 0.12 μm or less, and a fineferromagnetic hexagonal ferrite powder having an average particle sizeof 0.10 μm or less, high φm can be obtained thereby higher capacity andhigher density of the magnetic recording medium can be attained.Durability can be improved by high packing density of an inorganicpowder. Ultra-fine granulation of powders (a ferromagnetic powder and anonmagnetic powder) in the above item (5) is described. By the use of afine ferromagnetic powder, specifically a fine ferromagnetic metalpowder having an average long axis length of preferably 0.15 μm or less,more preferably 0.12 μm or less, and a fine ferromagnetic hexagonalferrite powder having an average particle size of 0.10 μm or less, inparticular, with the case of a ferromagnetic metal powder, by theultra-fine granulation of the powder such as an average long axis lengthof 0.10 μm or less, an acicular ratio of from 4.0 to 9.0, and acrystallite size of from 80 Å to 180 Å, higher packing density andhyper-smoothing of the magnetic layer can be attained, thereby highercapacity and higher density of the magnetic recording medium can beattained. Stabilization of head touch in the above item (6) isdescribed. Stabilization of head touch can be contrived by anappropriate strength, flexibility and smoothness of the magneticrecording medium as a whole, thereby higher capacity and higher densityof the magnetic recording medium can be attained stably even at highspeed running and high rotation rate. Dimensional stability andservomechanism in the above item (7) is described. For example, when thesupport has a thermal shrinkage factor of 0.5% or less both at 100° C.for 30 minutes and at 80° C. for 30 minutes in every direction ofin-plane of the support, and a temperature expansion coefficient of from10⁻⁴ to 10⁻⁸/° C. in every direction of in-plane of the support,dimensional stability of the support can be obtained, thereby highercapacity and higher density of the magnetic recording medium can beattained stably even at high speed running and high rotation rate, andimprovement of thermal shrinkage factors of the magnetic layer and thesupport in the above item (8) can also achieved. With respect to thefunctions of a lubricant at high temperature and low temperature in theabove item (9), desired lubricating functions at both high temperatureand low temperature can be obtained by selecting and combining variouslubricants described above based on specific concepts.

In the field of personal computers where the tendency of multimedia hasbeen increasingly progressed, high capacity recording media haveattracted public attentions in place of conventional floppy discs and,for example, ZIP disc, has been on sale from IOMEGA CORP., U.S.A. Thisis a recording medium comprising a lower layer and a magnetic thin layerdeveloped by the present inventors using ATOMM® (Advanced Super ThinLayer & High Output Metal Media Technology), and products of 3.7 incheswith the recording capacity of 100 MB or more are on the market. Thecapacity of from 100 to 120 MB discs is almost equal to the capacity ofMO (3.5 inches), i.e., one disc has the capacity of recording newspaperarticles of from seven to eight month period. A transfer rate indicatingwrite/readout time of data is 2 MB or more per a second, which is equalto a hard disc, and the working speed is 20 times of conventional floppydiscs and more than 2 times of the MO, therefore, extremelyadvantageous. In addition, as this recording medium comprising a lowerlayer and a magnetic thin layer is the same coating type medium asfloppy discs used at present, mass production is feasible, accordinglyinexpensive as compared with hard discs and the Mo.

As a result of eager investigations based on the knowledge on thesemedia, the present inventors have achieved the present invention of amagnetic recording medium, in particular, a disc-like magnetic recordingmedium, in the recording capacity system of areal recording density offrom 0.17 to 2 G bit/inch², preferably from 0.2 to 2 G bit/inch², morepreferably from 0.35 to 2 G bit/inch², and φm of preferably from10.0×10⁻³ to 1.0×10⁻³ emu/cm², particularly preferably from 8.0×10⁻³ to1.0×10⁻³ emu/cm², which has markedly high recording capacity as comparedwith the above ZIP disc and the MO (3.5 inches). This recording mediumalso has high capacity, high density characteristics and excellentdurability which could never be achieved by any products known in theworld and, in particular, the error rate in high density recordingregion is noticeably improved, and this is the invention applicable to amagnetic tape, e.g., a computer tape.

The magnetic recording medium of the present invention comprises anultra-thin magnetic layer containing a magnetic powder of ultra-fineparticles excellent in high output and high dispersibility, and a lowerlayer containing a spherical or acicular inorganic powder, and by thusreducing the thickness of the magnetic layer, a magnetic force offset inthe magnetic layer can be reduced, the output in a high frequency regioncan be markedly increased and, further, overwriting characteristics canbe improved. By the improvement of a magnetic head, the effect of theultra-thin magnetic layer can be further exhibited by the combined usewith a narrow gap head and digital recording characteristics can beimproved.

The upper magnetic layer is a thin layer having a thickness ofpreferably from 0.05 to 0.30 μm, more preferably from 0.05 to 0.25 μm,so as to match the performance required from the magnetic recordingsystem and the magnetic head of high density recording. Such a uniformand ultra-thin magnetic layer is attained by high dispersion and highpacking density realized by the combined use of a fine magnetic powderand nonmagnetic powder with a dispersant and a high dispersible binder.The magnetic powders used are preferably magnetic powders capable ofachieving high output, excellent in high dispersibility and highrandamizing property for inducing suitabilities of high capacity floppydiscs and computer tapes as far as possible. That is, high output andhigh durability can be attained by the use of ferromagnetic metalpowders of extremely fine particles which are capable of achieving highoutput, in particular, having an average long axis length of preferably0.15 μm or less, more preferably 0.12 μm or less, or ferromagnetichexagonal ferrite powders having an average particle size of 0.10 μm orless, in particular, by the use of ferromagnetic metal powders having along axis length of 0.1 μm or less and a crystallite size of from 80 to180 Å, containing a large amount of Co, and further Al, Si, Y and Nd asa sintering preventing agent. For the realization of a high transferrate, running stability and durability during high speed rotation can beensured making use of a three dimensional network binder system suitablefor an ultra-thin magnetic layer. A composite lubricant capable ofmaintaining the effect thereof during use under various temperature andhumidity conditions and in high rotation use can be incorporated intoupper and lower layers and, further, with making the lower layer have arole of the tank of the lubricant so as to be able to always supply anappropriate amount of the lubricant to the upper magnetic layer tothereby heighten the durability of the upper magnetic layer to improvethe reliance. Cushioning effect of the lower layer can bring about goodhead touch and stable running property.

A high transfer rate is required in a high capacity recording system,e.g., a transfer rate of 1.4 MB/sec. in Zip and a maximum transfer rateof 3.6 MB/sec. in HiFD®. For that sake, it is necessary that therotation speed of a magnetic disc for a high capacity recording systemshould be taken up one or more places as compared with conventional FDsystems. Specifically, the rotation speed of a magnetic disc ispreferably 1,800 rpm or more, more preferably 3,000 rpm or more. Forexample, the rotation speed of a magnetic disc is 2,968 rpm in Zip and3,600 rpm in HiFD®. In other systems, it is estimated that the rotationspeed of a magnetic disc is 5,400 rpm and the transfer rate is 7.5MB/sec when a recording capacity is 650 MB (0.65 GB). Recording trackdensity is improved with the increase of capacity/density of magneticrecording. In general, a servo recording area is provided on a medium toensure traceability of a magnetic head against a recording track. In themagnetic recording medium according to the present invention, a basewhose dimensional stability is isotropically heightened is preferablyused as the support base, thereby further stabilization of thetraceability is devised. The smoothness of the magnetic layer can befurther improved by using a hyper-smooth base.

The increment of density of magnetic recording of a disc-like magneticrecording medium requires the improvement of linear recording densityand track density. Characteristics of a support are important factorsfor the improvement of track density. The dimensional stability of asupport base, in particular, isotropy, is considered in the recordingmedium according to the present invention. Servo recording is anindispensable technique in recording/reproduction of high track density,but the improvement can be contrived from the medium side by making thesupport base isotropic as far as possible.

Advantages of changing the magnetic layer of the present invention froma monolayer (i.e., a single layer) to the ATOMM® structure are thoughtto be as follows.

(1) Improvement of electromagnetic characteristics by the thin layerstructure of the magnetic layer;

(2) Improvement of durability by stable supply of lubricants;

(3) High output by smoothing the upper magnetic layer; and

(4) Easiness of imparting required functions by functional separation ofthe magnetic layer.

These functions cannot be sufficiently attained only by making themagnetic layer a multilayer structure. For constituting a multilayerstructure, a successive multilayer system comprising successivelyconstituting the layers is generally used. In this system, the lowerlayer is coated, cured or dried, then the upper magnetic layer is coatedin the same way, cured, and surface-treated. In the case of a floppydisc (FD), as different from a magnetic tape, the same treatments areconducted on both surface sides. After a coating step, a disc undergoesa slitting step, a punching step, a shell incorporation step, and acertifying step, thus a final product is completed. Simultaneous coatingor successive coating of coating the upper magnetic layer while thelower layer is still wet is preferred in view of the productivity.

Electromagnetic characteristics can be widely improved by the thin layerstructure of the magnetic layer as follows.

(1) Improvement of the output in a high frequency region by theimprovement of characteristics of recording demagnetization;

(2) Improvement of overwriting characteristics; and

(3) Security of window margin.

Durability is an important factor for a magnetic recording disc. Inparticular, for realizing a high transfer rate, it is necessary that therotation speed of a magnetic disc should be taken up one or more placesas compared with conventional FD systems, and security of the durabilityof a magnetic disc is an important problem when the magnetic disc issliding with a magnetic head and parts in a cartridge at a high speed.For improving durability of a disc, there are means such as a binderformulation to increase the film strength of a disc per se, and alubricant formulation to maintain a sliding property of a disc with amagnetic head. In the magnetic recording medium according to the presentinvention, a three dimensional network binder system which has shownactual results in conventional FD systems is used in the binderformulation by being modified.

In the present invention, lubricants are used in combination of aplurality of kinds respectively exhibiting superior effects in varioustemperature and humidity conditions under which they are used, and eachlubricant exhibits its function in different temperature (lowtemperature, room temperature, high temperature) and humidity (lowhumidity, high humidity) atmospheres, thereby totally stable lubricationeffect can be maintained.

By using two layer structure, the durability of the upper magnetic layercan be heightened with making the lower layer have a role of the tank ofa lubricant capable of always supplying an appropriate amount of alubricant to the upper magnetic layer. There is a limit on the amount ofa lubricant which can be contained in the ultra-thin magnetic layer.Simple reduction of the thickness of the magnetic layer causes thereduction of the absolute amount of a lubricant, and it follows thatrunning durability is deteriorated. In this case, it was difficult towell balance the thickness of the magnetic layer with the amount of thelubricant. The improvement of electromagnetic characteristics could becompatible with the improvement of durability by imparting differentfunctions to the upper layer and the lower layer and making up for eachother. This functional separation was particularly effective in a systemwhere a medium was slid on a magnetic head at a high speed.

In addition to the maintaining function of a lubricant, a controllingfunction of surface electrical resistance can be imparted to the lowerlayer. For controlling electrical resistance, in general, a solidelectrically conductive material such as a carbon black is added to amagnetic layer in many cases. Such a material not only restricts theincrease of the packing density of magnetic powders but also influencesthe surface roughness of the magnetic layer as the thickness of themagnetic layer becomes thinner. Incorporation of electrically conductivematerials in the lower layer can eliminate these defects.

With the progress of multimedia in society, needs for image recordinghave been increased more and more not only in the industry but also ingeneral homes. The high capacity magnetic recording medium according tothe present invention has capabilities capable of sufficientlyresponding to demands such as function/cost as a medium for recordingimages, as well as data such as letters and figures. The high capacitymagnetic recording medium according to the present invention is based onthe coating type magnetic recording medium which has shown actualresults and ensures reliability for a long period of time and isexcellent in cost performance.

The present invention has been attained for the first time by heaping upthe above various factors, and making them worked synergistically andorganically. The thus-obtained magnetic recording medium by adopting orrejecting and combining every technique has capability applicable to,e.g., HiFD®, which has been developed by joint development by Fuji PhotoFilm Co., Ltd. with Sony Corp. HiFD® has been developed to meet thedemand for a new data recording system of high performance having highcapacity and high data transfer rate with the rapid development ofinformation processing capability of personal computers in recent yearsand sharp increase of throughput to be dealt with. On the other hand,3.5 inch type floppy discs of the present have prevailed worldwide aseasily usable recording media. HiFD® has been developed as a new systemwhich can read out and reuse accumulated massive data using these discseven after this. HiFD® for 3.5 inch type floppy disc is a high capacityfloppy disc system of the next generation which has high capacity of 200MB, high transfer rate of 3.6 MB/sec, and capable of realizingsubordination transposition capable of recording/reproduction with 3.5inch type floppy discs of the present. High capacity of 200 MB of HiFD®has been realized by an ultra-thin layer coating type metal disc newlydeveloped and by the adoption of a dual discrete gap head having both anarrow gap for high density recording and a broad gap for a 3.5 inchtype floppy disc of the present, which can easily process data file ofhuge volume such as an image and a sound. HiFD® has realized a hightransfer rate of a maximum of 3.6 MB/sec. as compared with a transferrate of 0.06 MB/sec. of conventional 3.5 inch type floppy discs (2HD)due to high linear recording density and high speed disc rotation suchas 3,600 rpm, which is high speed processing of about 60 times ascompared with conventional systems. Further, by the adoption of afloatation type dual discrete gap head and, at the same time, by the useof a linear type voice coil motor as a driving motor of the head, HiFD®has achieved high speed random access of about 3 to 4 times as comparedwith conventional 3.5 inch type floppy disc drives. The floatation typedual discrete gap head, similar to a hard disc, floats by disc rotationand does not contact with the disc during recording/reproduction leadingto long lifetime and high reliability. In addition, by the dual discretegap head, subordination transposition capable of recording/reproductionwith 3.5 inch type floppy discs of the present has been realized.Further, abrasion of a disc can be reduced by the integration of a newmechanism capable of soft head loading, and high reliability can beattained by the loading of an error correcting function. The magneticrecording medium according to the present invention has been developedto be applicable to such a high capacity floppy disc system of the nextgeneration which has high capacity of 200 MB, high transfer rate of 3.6MB/sec, and has realized subordination transposition capable ofrecording/reproduction with 3.5 inch type floppy discs of the present.

Magnetic Layer

The lower layer and the ultrathin magnetic layer of the magneticrecording medium according to the present invention may be provided oneither one side of the support or may be provided on both sides. Theupper magnetic layer may be coated while the lower layer coated is stillwet (W/W coating) or may be coated after the lower layer coated is dried(W/D coating). Simultaneous or successive wet on wet coating ispreferred in view of the productivity but in the case of a disc-likemedium, wet on dry coating can be sufficiently used. In the multilayerconstruction according to the present invention, as the upper layer andthe lower layer can be formed simultaneously or successively (with W/Wcoating), a surface treatment step, e.g., a calendering step, can beutilized effectively and surface roughness of the upper magnetic layercan be improved even the layer is an ultrathin layer. The coercive force(Hc) of the magnetic layer is essential to be 1,800 Oe or more, and themaximum magnetic flux density (Bm) of magnetic metal powders ispreferably from 2,000 to 5,000 G and of barium ferrite powders is from1,000 to 3,000 G.

Ferromagnetic Metal Powder

The ferromagnetic powders which can be used in the present invention arepreferably ferromagnetic alloy powders containing α-Fe as a maincomponent. These ferromagnetic powders which can be preferably used inthe upper magnetic layer of the present invention may contain, inaddition to the prescribed atoms, the following atoms, e.g., Al, Si, S,Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au,Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr and B. In particular,it is preferred to contain at least one of Al, Si, Ca, Y, Ba, La, Nd,Co, Ni and B, in addition to α-Fe, and more preferably at least one ofCo, Y and Al in addition to α-Fe. The content of Co is preferably from 0to 40 atomic %, more preferably from 15 to 35 atomic %, and mostpreferably from 20 to 35 atomic %, the content of Y is preferably from1.5 to 12 atomic %, more preferably from 3 to 10 atomic %, and mostpreferably from 4 to 9 atomic %, and the content of Al is preferablyfrom 1.5 to 12 atomic %, more preferably from 3 to 10 atomic %, and mostpreferably from 4 to 9 atomic %, each based on Fe. These ferromagneticpowders may be previously treated with the later described dispersant,lubricant, surfactant, and antistatic agent before dispersion. Specificexamples thereof are disclosed in JP-B-44-14090, JP-B-45-18372,JP-B-47-22062, JP-B-47-22513, JP-B-46-28466, JP-B-46-38755,JP-B-47-4286, JP-B-47-12422, JP-B-47-17284, JP-B-47-18509,JP-B-47-18573, JP-B-39-10307, JP-B-46-39639, U.S. Pat. Nos. 3,026,215,3,031,341, 3,100,194, 3,242,005, and 3,389,014.

Ferromagnetic alloy powders may contain a small amount of a hydroxide oran oxide. Ferromagnetic alloy powders can be prepared by well-knownmethods, such as a method comprising reducing a composite organic acidsalt (mainly an oxalate) with a reducing gas, e.g., hydrogen; a methodcomprising reducing iron oxide with a reducing gas, e.g., hydrogen, toobtain Fe or Fe—Co particles; a method comprising pyrolysis of a metalcarbonyl compound; a method comprising adding to an aqueous solution ofa ferromagnetic metal a reducing agent, e.g., sodium boronhydride,hypo-phosphite, or hydrazine, to conduct reduction; and a methodcomprising evaporating a metal in a low pressure inert gas to obtain afine powder. The thus-obtained ferromagnetic alloy powders which aresubjected to well-known gradual oxidization treatment can be used in thepresent invention, e.g., a method comprising immersing powders in anorganic solvent, then drying; a method comprising immersing powders inan organic solvent, then charging an oxygen-containing gas to form oxidefilms on the surfaces thereof and drying; and a method comprisingforming oxide films on the surfaces of the powders by regulating partialpressure of an oxygen gas and an inert gas without using an organicsolvent.

Ferromagnetic powders which can be preferably used in the magnetic layeraccording to the present invention have a specific surface area(S_(BET)) as measured by the BET method of from 45 to 80 m²/g,preferably from 50 to 70 m²/g. When S_(BET) is less than 45 m²/g, noiseincreases and when more than 80 m²/g, good surface property is obtainedwith difficulty, which is not preferred. Ferromagnetic powders which canbe preferably used in the magnetic layer according to the presentinvention have a crystallite size of generally from 80 to 180 Å,preferably from 100 to 180 Å, and more preferably from 110 to 175 Å. Thelength of the long axis of ferromagnetic powders is generally from 0.01to 0.25 μm, preferably from 0.03 to 0.15 μm, and more preferably from0.03 to 0.12 μm. Ferromagnetic powders preferably have an acicular ratioof from 3.0 to 15.0, more preferably from 4.0 to 12.0, and particularlypreferably from 4.0 to 9.0. Ferromagnetic metal powders have asaturation magnetization (σ_(s)) of generally from 100 to 180 emu/g,preferably from 110 to 170 emu/g, and more preferably from 125 to 160emu/g. Ferromagnetic metal powders have a coercive force (Hc) ofpreferably from 1,700 to 3,500 Oe, and more preferably from 1,800 to3,000 Oe.

Ferromagnetic metal powders preferably have a water content of from 0.01to 2%. The water content of ferromagnetic metal powders is preferablyoptimized by selecting the kinds of binders. The pH of ferromagneticmetal powders is preferably optimized by the combination with the binderto be used. The pH range is from 4 to 12, preferably from 6 to 10.Ferromagnetic metal powders may be surface-treated with Al, Si, P oroxides thereof, if necessary. The amount thereof is from 0.1 to 10%based on the ferromagnetic metal powders. Adsorption of a lubricant,e.g., fatty acid, becomes 100 mg/m² or less by conducting a surfacetreatment, which is, therefore, preferred. Soluble inorganic ions (e.g.,Na, Ca, Fe, Ni, Sr, etc.) are sometimes contained in ferromagnetic metalpowders. It is preferred substantially not to contain such solubleinorganic ions but the properties of ferromagnetic metal powders are notparticularly affected if the content is 200 ppm or less. Ferromagneticmetal powders for use in the present invention preferably have lessvoids and the value thereof is 20% by volume or less, more preferably 5%by volume or less. The shape of ferromagnetic metal powders is notparticularly limited, and any shape such as an acicular shape, anellipsoidal shape or a spindle shape may be used so long as it satisfiesthe above-described properties as to particle sizes. Switching FieldDistribution (SFD) of a ferromagnetic metal powder itself is preferablysmall, preferably 0.8 or less. It is necessary to make Hc distributionof ferromagnetic metal powders narrow. When the SFD is 0.8 or less,electromagnetic characteristics are excellent, high output can beobtained, reversal of magnetization becomes sharp and peak shift isless, therefore, suitable for high density digital magnetic recording.For achieving small Hc distribution, making particle size distributionof goethite in ferromagnetic metal powders good and preventing sinteringare effective methods.

Hexagonal Ferrite Powder

Examples of hexagonal ferrite which can be preferably used in theupper(most) magnetic layer in the present invention include substitutionproducts of barium ferrite, strontium ferrite, lead ferrite and calciumferrite and Co substitution products. Specifically, magnetoplumbite typebarium ferrite and strontium ferrite, magnetoplumbite type ferritehaving covered the particle surfaces with spinel, magnetoplumbite typebarium ferrite and strontium ferrite partially containing spinel phase,etc., are exemplified. Hexagonal ferrite powders may contain, inaddition to the prescribed atoms, the following atoms, e.g., Al, Si, S,Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg,Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge and Nb. In general,those containing the following elements can be used, e.g., Co—Zn, Co—Ti,Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, Nb—Zn, etc. Accordingto starting materials and producing processes, specific impurities maybe contained. The hexagonal ferrite has an average hexagonal tabulardiameter of from 10 to 200 nm, preferably from 10 to 100 nm, andparticularly preferably from 10 to 80 nm.

When reproduction is conducted using a magneto resistance head forincreasing track density, it is necessary to reduce noise, accordinglythe average tabular diameter is preferably 40 nm or less, but if it issmaller than 10 nm, stable magnetization cannot be obtained due tothermal fluctuation. While when it is more than 200 nm, noise increases,therefore, both of such particle diameters are not suitable for highdensity recording. A tabular ratio (tabular diameter/tabular thickness)is preferably from 1 to 15, more preferably from 1 to 7. If a tabularratio is small, the packing density in a magnetic layer becomes high,which is preferred but satisfactory orientation cannot be obtained. If atabular ratio is more than 15, noise increases due to stacking amongparticles. The specific surface area (S_(BET)) measured by the BETmethod of the particles having diameters within this range is from 10 to200 m²/g. Specific surface areas nearly coincide with the valuesobtained by arithmetic operations from tabular diameters and tabularthicknesses. Distribution of tabular diameter/tabular thickness isgenerally preferably as narrow as possible. It is difficult to showspecific surface area distributions in numerical values butdistributions can be compared by measuring TEM photographs of 500particles selected randomly. Distributions are in many cases not regulardistribution, but when expressed by the standard deviation to theaverage diameter from calculation, a/average diameter is from 0.1 to2.0. For obtaining narrow particle size distribution, it is efficient tomake a particle forming reaction system homogeneous as far as possible,and particles formed are subjected to distribution-improving treatmentsas well. For example, a method of selectively dissolving ultrafineparticles in an acid solution is also known. Coercive force (Hc) ofgenerally from about 500 to about 5,000 Oe measured in magnetic powderscan be produced. Higher Hc is advantageous for high density recordingbut it is restricted by capacities of recording heads. The magneticpowders according to the present invention have Hc of from about 1,700to about 4,000 Oe, preferably from 1,800 to 3,500 Oe. When saturationmagnetization of the head is more than 1.4 tesla, Hc of 2,000 Oe or moreis preferred. Hc can be controlled by particle diameters (tabulardiameter/tabular thickness), kinds and amounts of elements contained,substitution sites of elements, and reaction conditions of particleformation. Saturation magnetization (σ_(s)) is from 40 to 80 emu/g.σ_(s) is preferably higher but it has inclination of becoming smaller asparticles become finer. For the improvement of σ_(s), it is well knownto make composite of magnetoplumbite ferrite with spinel ferrite, toselect kinds and amounts of elements to be contained, or W typehexagonal ferrite can also be used. Further, when magnetic powders aredispersed, particle surfaces of magnetic powders may be treated withsubstances compatible with the dispersion media and the polymers.Inorganic or organic compounds are used as a surface treating agent. Forexample, oxides or hydroxides of Si, Al, P, etc., various kinds ofsilane coupling agents, and various kinds of titanium coupling agentsare representative examples. The amount of these surface treating agentsis from 0.1 to 10% based on the amount of the magnetic powder. The pH ofmagnetic powders is also important for dispersion, and is in generalfrom 4 to 12. The optimal value is dependent upon the dispersion mediumand the polymer. Taking chemical stability and storage stability ofmagnetic media into consideration, pH of from 6 to 11 or so is selected.The water content in the magnetic powder also affects dispersion. Theoptimal value is dependent upon the dispersion medium and the polymer,and the water content of from 0.01 to 2.0% is selected in general.Producing methods of hexagonal ferrite include the following and any ofthese methods can be used in the present invention: (1) a glasscrystallization method in which metal oxides which substitute bariumoxide, iron oxide and iron, and boron oxide, etc., as a glass formingmaterial are mixed so as to make a desired ferrite composition, melted,and then quenched to obtain an amorphous product, the obtained productis reheat-treated, washed and then pulverized to obtain a barium ferritecrystal powder, (2) a hydrothermal reaction method in which a solutionof barium ferrite composition metal salts is neutralized with an alkali,byproducts are removed followed by liquid phase heating at 100° C. ormore, washed, dried and then pulverized to obtain a barium ferritecrystal powder, and (3) a coprecipitation method in which a solution ofbarium ferrite composition metal salts is neutralized with an alkali,byproducts are removed followed by drying, treated at 1,100° C. or less,and then pulverized to obtain a barium ferrite crystal powder.

Nonmagnetic Layer

The lower layer is described in detail below. Inorganic powderscontained in the lower layer of the present invention are nonmagneticpowders. They can be selected from the following inorganic compoundssuch as metal oxide, metal carbonate, metal sulfate, metal nitride,metal carbide, metal sulfide, etc. Examples of inorganic compounds areselected from the following compounds and they can be used alone or incombination, e.g., α-alumina having an alpha-conversion rate of 90% ormore, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide,cerium oxide, α-iron oxide, hematite, goethite, corundum, siliconnitride, titanium carbide, titanium oxide, silicon dioxide, stannicoxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride,zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, andmolybdenum disulfide. Of these compounds, particularly preferred aretitanium dioxide, zinc oxide, iron oxide and barium sulfate because theyhave small particle size distribution and various means for impartingfunctions, and more preferred are titanium dioxide and α-iron oxide.These nonmagnetic powders preferably have an average particle size offrom 0.005 to 2 μm. If desired, a plurality of nonmagnetic powders eachhaving a different average particle size may be combined, or a singlenonmagnetic powder having a broad particle size distribution may beemployed so as to attain the same effect as such a combination. Aparticularly preferred particle size of nonmagnetic powders is from 0.01to 0.2 μm. In particular, when the nonmagnetic powder is a granularmetal oxide, the average particle size thereof is preferably 0.08 μm orless, and when it is an acicular metal oxide, the average long axislength thereof is preferably 0.3 μm or less, more preferably 0.2 μm orless. Nonmagnetic powders for use in the present invention have a tapdensity of from 0.05 to 2 g/ml, preferably from 0.2 to 1.5 g/ml; a watercontent of from 0.1 to 5% by weight, preferably from 0.2 to 3% byweight, and more preferably from 0.3 to 1.5% by weight; a pH value offrom 2 to 11, particularly preferably between 5.5 and 10; a specificsurface area (S_(BET)) of from 1 to 100 m²/g, preferably from 5 to 80m²/g, and more preferably from 10 to 70 m²/g; a crystallite size of from0.004 to 1 μm, and more preferably from 0.04 to 0.1 μm; an oilabsorption amount using DBP (dibutyl phthalate) of from 5 to 100 ml/100g, preferably from 10 to 80 ml/100 g, and more preferably from 20 to 60ml/100 g; and a specific gravity of from 1 to 12, preferably from 3 to6. The shape of nonmagnetic powders may be any of acicular, spherical,polyhedral, or tabular shape. Nonmagnetic powders preferably have aMohs' hardness of from 4 to 10. The SA (stearic acid) absorption amountof nonmagnetic powders is from 1 to 20 μmol/m², preferably from 2 to 15μmol/m², and more preferably from 3 to 8 μmol/m². The pH thereof ispreferably between 3 and 6. The surfaces of these nonmagnetic powdersare preferably covered with Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂O₃, ZnO orY₂O₃. Preferred in the point of dispersibility are Al₂O₃, SiO₂, TiO₂ andZrO₂, and more preferred are Al₂O₃, SiO₂ and ZrO₂. They can be used incombination or alone. A method in which the surface treatment may beperformed by coprecipitation, alternatively, surface treatment ofparticles may be previously performed to be covered with alumina in thefirst place, then the alumina-covered surface is covered with silica, orvice versa, according to purposes. The surface-covering layer may beporous layer, if necessary, but a homogeneous and dense surface isgenerally preferred.

Specific examples of nonmagnetic powders for use in the lower layeraccording to the present invention include HIT-100 (average particlesize: 0.11 μm), and ZA-G1 (manufactured by Sumitomo Chemical Co., Ltd.)as alumina, Nanotite (average particle size: 0.06 μm) (manufactured byShowa Denko Co., Ltd.), α-hematite DPN-250, DPN-250BX, DPN-245,DPN-270BX, DPN-550BX (average long axis length: 0.16 μm, average shortaxis length: 0.02 μm, axis ratio: 7.45), DPN-550RX (average long axislength: 0.16 μm, average short axis length: 0.02 μm, axis ratio: 7.45),and DPN-650RX (manufactured by Toda Kogyo Co., Ltd.), α-hematite α-40(manufactured by Titan Kogyo Co., Ltd.), α-hematite E270, E271, E300 andE303 (manufactured by Ishihara Sangyo Kaisha Ltd.) as iron oxide,titanium oxide TTO-51B (average particle size: from 0.01 to 0.03 μm),TTO-55A (average particle size: from 0.03 to 0.05 μm), TTO-55B (averageparticle size: from 0.03 to 0.05 μm), TTO-55C (average particle size:from 0.03 to 0.05 μm), TTO-55S (average particle size: from 0.03 to 0.05μm), TTO-55D (average particle size: from 0.03 to 0.05 μm), and SN-100(manufactured by Ishihara Sangyo Kaisha Ltd.), titanium oxide STT-4D(average particle size: 0.013 μm), STT-30D (average particle size: 0.09em), STT-30 (average particle size: 0.12 μm), STT-65C (average particlesize: 0.12 μm) (manufactured by Titan Kogyo Co., Ltd.), titanium oxideMT-100S (average particle size: 0.015 μm), MT-100T (average particlesize: 0.015 μm), MT-150W (average particle size: 0.015 μm), MT-500B(average particle size: 0.035 μm), MT-600B (average particle size: 0.050μm), MT-100F, and MT-500HD (manufactured by Teika Co., Ltd.) as titaniumoxide, FINEX-25 (average particle size: 0.5 μm) (manufactured by SakaiChemical Industry Co., Ltd.) as zinc oxide, BF-1 (average particle size:0.05 μm), BF-10 (average particle size: 0.06 μm), BF-20 (averageparticle size: 0.03 μm), and ST-M (manufactured by Sakai ChemicalIndustry Co., Ltd.) as barium sulfate, DEFIC-Y and DEFIC-R (manufacturedby Dowa Mining Co., Ltd.), AS2BM and TiO₂ P25 (manufactured by NipponAerosil Co., Ltd.), and 100A, 500A and calcined products thereof(manufactured by Ube Industries, Ltd.). Particularly preferrednonmagnetic powders are titanium dioxide and α-iron oxide.

Preparation of α-iron oxide (hematite) is performed as follows. α-Fe₂O₃powders are obtained from acicular goethite particles as precursorparticles. Acicular goethite particles can be produced by any of thefollowing methods.

(1) A method in which an aqueous alkali hydroxide solution is added toan aqueous ferrous salt solution in equivalent or more amount to therebyprepare a suspension having pH of 11 or more containing ferroushydroxide colloid, then an oxygen-containing gas is introduced to thesuspension obtained at 80° C. or less to form acicular goethiteparticles by the oxidation reaction of ferrous ions;

(2) A method in which an aqueous ferrous salt solution is reacted withan aqueous alkali carbonate solution to thereby prepare a suspensioncontaining FeCO₃, then an oxygen-containing gas is introduced to thesuspension obtained to form spindle-like goethite particles by theoxidation reaction of ferrous ions;

(3) A method in which an aqueous alkali hydroxide solution or an aqueousalkali carbonate solution is added to an aqueous ferrous salt solutionin the amount of less than equivalent, thereby an aqueous ferrous saltsolution containing ferrous hydroxide colloid is prepared, then anoxygen-containing gas is introduced to the aqueous ferrous salt solutionobtained to form acicular goethite nucleus particles by the oxidationreaction of ferrous ions, thereafter an aqueous alkali hydroxidesolution is added to the aqueous ferrous salt solution containing theacicular goethite nucleus particles in the amount of equivalent or morebased on Fe²⁺ in the aqueous ferrous salt solution, then again anoxygen-containing gas is introduced to the aqueous ferrous salt solutionto grow the acicular goethite nucleus particles; and

(4) A method in which an aqueous alkali hydroxide solution or an aqueousalkali carbonate solution is added to an aqueous ferrous salt solutionin the amount of less than equivalent, thereby an aqueous ferrous saltsolution containing ferrous hydroxide colloid is prepared, then anoxygen-containing gas is introduced to the aqueous ferrous salt solutionobtained to form acicular goethite nucleus particles by the oxidationreaction of ferrous ions, thereafter the acicular goethite nucleusparticles are grown in an acidic or neutral region.

Further, different kinds of elements such as Ni, Zn, P or Si, which aregenerally added to the reaction solution during the goethiteparticle-forming reaction for improving the properties of the powder,may be added. Acicular α-Fe₂O₃ particles can be obtained by dehydratingacicular goethite particles, which are precursor particles, in the rangeof 200 to 500° C. and further, if necessary, annealing the particles byheat treatment at 350 to 800° C. A sintering inhibitor such as P, Si, B,Zr or Sb may be adhered to the surface of acicular goethite particles tobe dehydrated or annealed. The reason why annealing by heat treatment at350 to 800° C. is conducted is because it is preferred to fill the voidswhich have occurred on the surface of acicular α-Fe₂O₃ particlesobtained by the dehydration by melting the extreme surfaces of particlesto obtain smooth surfaces.

The α-Fe₂O₃ powder for use in the present invention can be obtained bydispersing acicular α-Fe₂O₃ particles obtained by dehydration orannealing in an aqueous solution to make a suspension, adding Alcompounds to the suspension obtained and adjusting the pH, covering thesurface of acicular α-Fe₂O₃ particles with the above Al compounds,filtering, washing, drying, pulverizing and, if necessary, performingother treatments such as deaeration, compaction and the like. Aluminumsalt such as aluminum acetate, aluminum sulfate, aluminum chloride, andaluminum nitrate, and aluminic acid alkali salt such as sodium aluminatecan be used as the aluminum compound to be used. In this case, theaddition amount of the Al compound is from 0.01 to 50% by weight interms of Al based on the α-Fe₂O₃ powder. If the content is less than0.01% by weight, dispersion in the binder resin is insufficient and ifit exceeds 50% by weight, Al compounds suspending around surfaces ofparticles unfavorably interact with each other. The nonmagnetic powderfor use in the lower layer according to the present invention may becovered with one or two or more selected from the group consisting of P,Ti, Mn, Ni, Zn, Zr, Sn and Sb, as well as Si compounds, together with Alcompounds. The content of these compounds used together with Alcompounds is each from 0.01 to 50% by weight based on the α-Fe₂O₃powder. If the content is less than 0.01% by weight, the improvement ofdispersibility by the addition can hardly be obtained, and if it exceeds50% by weight, Al compounds suspending around surfaces of particlesunfavorably interact with each other.

The producing method of titanium dioxide is as follows. The producingmethod of titanium dioxide mainly comprises a sulfuric acid process anda chlorine process. A sulfuric acid process comprises digesting raw oresof ilmenite with sulfuric acid and extracting Ti and Fe as sulfate. Ironsulfate is removed by crystallization-separation, the resulting titanylsulfate solution is purified by filtration, water-containing titaniumoxide is precipitated by thermal hydrolysis, the precipitated product isfiltrated and washed, impurities are removed by washing, then a particlesize-adjusting agent is added and calcined at 80 to 1,000° C., therebycrude titanium oxide is obtained. A rutile type and an anatase type areseparated by the kind of nucleating agent added at hydrolysis. Thiscrude titanium oxide is pulverized, graded, and surface treated. In achlorine process, natural rutile and synthetic rutile are used as rawores. Ores are chlorinated in a high temperature reduction state, Tibecomes TiCl₄ and Fe becomes FeCl₂, and the iron oxide solidified bycooling is separated from the liquid TiCl₄. The crude TiCl₄ obtained ispurified by fraction, then a nucleating agent is added thereto andreacted with oxygen instantaneously at 1,000° C. or more, thereby crudetitanium oxide is obtained. The finishing method for imparting to thecrude titanium oxide formed in the oxidation decomposition process theproperty of pigment is the same as in the sulfuric acid process.

After the above titanium oxide material is dry-ground, water and adispersant are added thereto, grains are wet-ground, and coarse grainsare classified by means of a centrifugal separator. Subsequently, a finegrain slurry is put in a surface treatment bath and surface coveringwith metal hydroxide is conducted here. In the first place, apredetermined amount of an aqueous solution of salts such as Al, Si, Ti,Zr, Sb, Sn, Zn is added to the tank, acid or alkali is added toneutralize the solution, and surfaces of titanium oxide particles arecovered with the hydroxide produced. The water-soluble salts by-producedare removed by decantation, filtration and washing, the pH of the slurryis adjusted finally and filtrated, and washed with pure water. Thewashed cake is dried using a spray drier or a band drier. The dried cakeis finally ground by jet milling, thereby the final product is obtained.

Besides the water system, it is also possible to perform surfacetreatment by introducing AlCl₃ and SiCl₄ vapor to the titanium oxidepowder, then water vapor is flowed to conduct surface treatment with Aland Si.

By the incorporation of carbon blacks into the lower layer, a desiredmicro Vickers' hardness can be obtained in addition to the well-knowneffects of reducing surface electrical resistance (Rs) and lighttransmittance. Further, it is also possible to obtain the effect ofstocking a lubricant by the incorporation of carbon blacks into thelower layer. Furnace blacks for rubbers, thermal blacks for rubbers,carbon blacks for coloring, acetylene blacks, etc. can be used therefor.Carbon blacks used in the lower layer should optimize the followingcharacteristics by the desired effects and sometimes more effects can beobtained by the combined use.

Carbon blacks for use in the lower layer according to the presentinvention have a specific surface area (S_(BET)) of from 100 to 500m²/g, preferably from 150 to 400 m²/g, a DBP oil absorption of from 20to 400 ml/100 g, preferably from 30 to 400 ml/100 g, an average particlesize of from 5 to 80 mμ, preferably from 10 to 50 mμ, and morepreferably from 10 to 40 mμ, and a small amount of carbon blacks havingan average particle size of larger than 80 mμ may be contained in thelower layer. Carbon blacks for use in the lower layer have pH of from 2to 10, a water content of from 0.1 to 10%, and a tap density of from 0.1to 1 g/ml. Specific examples of carbon blacks for use in the presentinvention include BLACKPEARLES 2000 (average particle size: 15 nm), 1400(average particle size: 13 nm), 1300 (average particle size: 13 nm),1100 (average particle size: 14 nm), 1000, 900 (average particle size:15 nm), 800, 880 and 700, L (average particle size: 24 nm), VULCAN XC-72(average particle size: 30 nm), and P (average particle size: 19 nm)(manufactured by Cabot Co., Ltd.), #3050B, #3150B, #3250B (averageparticle size: 30 nm), #3750B, #3950B (average particle size: 16 nm),#950 (average particle size: 16 nm), #650B, #970B, #850B (averageparticle size: 18 nm), MA-600 (average particle size: 18 nm), MA-230,#4000 and #4010 (manufactured by Mitsubishi Kasei Corp.), CONDUCTEX SC(average particle size: 17 nm), SC-U (average particle size: 20 nm), 975(average particle size: 20 nm), RAVEN 8800 (average particle size: 13nm), 8000 (average particle size: 13 nm), 7000 (average particle size:14 nm), 5750 (average particle size: 17 nm), 5250 (average particlesize: 19 nm), 5000 (average particle size: 12 nm), 3500 (averageparticle size: 16 nm), 2100 (average particle size: 17 nm), 2000(average particle size: 18 nm), 1800 (average particle size: 18 nm),1500 (average particle size: 18 nm), 1255 (average particle size: 23nm), 1250 (average particle size: 21 nm), and 1035 (average particlesize: 27 nm) (manufactured by Columbia Carbon Co., Ltd.), Ketjen BlackEC (average particle size: 30 nm) (manufactured by Akzo Co., Ltd.), and#80 (average particle size: 20 nm), #70 (average particle size: 27 nm),#60 (average particle size: 49 nm), #55 (average particle size: 68 nm),and Asahi Thermal (average particle size: 72 nm) (manufactured by AsahiCarbon Co., Ltd.). Carbon blacks having an average particle size oflarger than 80 mμ which may be used in the lower layer include #50(average particle size: 94 nm) and #35 (average particle size: 82 nm)(manufactured by Asahi Carbon Co., Ltd.). Carbon blacks for use in thepresent invention may previously be surface-treated with a dispersant,may be grafted with a resin, or a part of the surface thereof may begraphitized before use. Carbon blacks may be previously dispersed in abinder before addition to the coating solution. Carbon blacks can beused within the range not exceeding 50% by weight based on the aboveinorganic powders and not exceeding 40% by weight based on the totalweight of the nonmagnetic layer. These carbon blacks can be used aloneor in combination. Regarding carbon blacks for use in the presentinvention, for example, the disclosure in Handbook of Carbon Blacks(edited by Carbon Black Association of Japan) may be referred to.

Organic powders can be used in the lower layer according to the purpose.Examples of such organic powders include an acryl styrene resin powder,a benzoguanamine resin powder, a melamine resin powder, and aphthalocyanine pigment. In addition, at least one of a polyolefin resinpowder, a polyester resin powder, a polyamide resin powder, a polyimideresin powder, and a polyethylene fluoride resin powder can also be used.The producing methods thereof are disclosed in JP-A-62-18564 andJP-A-60-255827.

Binder

Binder resins, lubricants, dispersants, additives, solvents, dispersingmethods, etc., used for the magnetic layer described below can be usedin the lower layer and the backing layer. In particular, with respect tothe amounts and the kinds of binder resins, and the amounts and thekinds of additives and dispersants, well-known prior art techniquesregarding the magnetic layer can be applied to the lower layer accordingto the present invention.

Conventionally well-known thermoplastic resins, thermosetting resins,reactive resins and mixtures of these resins are used as a binder in thepresent invention. Thermoplastic resins having a glass transitiontemperature of from −100 to 150° C., a number average molecular weightof from 1,000 to 200,000, preferably from 10,000 to 100,000, and apolymerization degree of about 50 to 1,000 can be used in the presentinvention.

Examples thereof include polymers or copolymers containing as aconstituting unit the following compounds, such as vinyl chloride, vinylacetate, vinyl alcohol, maleic acid, acrylic acid, acrylate, vinylidenechloride, acrylonitrile, methacrylic acid, methacrylate, styrene,butadiene, ethylene, vinyl butyral, vinyl acetal and vinyl ether;polyurethane resins and various rubber resins. Examples of thermosettingresins and reactive resins usable in the present invention includephenol resins, epoxy resins, curable type polyurethane resins, urearesins, melamine resins, alkyd resins, acrylic reactive resins,formaldehyde resins, silicone resins, epoxy-polyamide resins, mixturesof polyester resins and isocyanate prepolymers, mixtures ofpolyesterpolyol and polyisocyanate, and mixtures of polyurethane andpolyisocyanate. Details on these resins are described in PlasticHandbook, published by Asakura Shoten. It is also possible to usewell-known electron beam curable type resins in each layer. Examples ofthese resins and producing methods are disclosed in detail inJP-A-62-256219. These resins can be used alone or in combination.Examples of preferred combinations include at least one selected fromvinyl chloride resins, vinyl chloride-vinyl acetate copolymers, vinylchloride-vinyl acetate-vinyl alcohol copolymers, and vinylchloride-vinyl acetate-maleic anhydride copolymers with polyurethaneresins, or combinations of these resins with polyisocyanate.

As polyurethane resins, those having well-known structures can be used,e.g., polyester polyurethane, polyether polyurethane, polyetherpolyester polyurethane, polycarbonate polyurethane, polyesterpolycarbonate polyurethane, polycaprolactone polyurethane, etc.Preferably, at least one polar group selected from the following groupsis introduced into the above binders by copolymerization or additionreaction for the purpose of further improving the dispersibility and thedurability, e.g., —COOM, —SO₃M, —OSO₃M, —P═O(OM)₂, —O—P═O(OM)₂ (whereinM represents a hydrogen atom, or an alkali metal salt group), —NR₂,—N⁺R₃ (R represents a hydrocarbon group), an epoxy group, —SH, or —CN.The content of the polar group is from 10⁻¹ to 10⁻⁸ mol/g, preferablyfrom 10⁻² to 10⁻⁶ mol/g. It is preferred that polyurethane resins haveat least one OH group at each terminal of polyurethane molecule, i.e.,two or more in total, in addition to the above polar groups. As OHgroups form three dimensional network structure by crosslinking with thepolyisocyanate curing agent, they are preferably contained in themolecule as many as possible. In particular, as the reactivity with thecuring agent is high, OH groups are preferably present at terminals ofthe molecule. It is preferred for polyurethane to have 3 or more OHgroups, particularly preferably 4 or more OH groups, at terminals of themolecule. When polyurethane is used in the present invention, thepolyurethane has a glass transition temperature of from −50 to 150° C.,preferably from 0 to 100° C., breaking extension of from 100 to 2,000%,breaking stress of from 0.05 to 10 kg/mm², and a yielding point of from0.05 to 10 kg/mm². Due to these physical properties, coated filmexhibiting good mechanical properties at high rotation rate ofpreferably 1,800 rpm or more, more preferably 3,000 rpm or more, can beobtained.

Specific examples of binders for use in the present invention includeVAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH,PKHJ, PKHC and PKFE (manufactured by Union Carbide Co., Ltd.), MPR-TA,MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM and MPR-TAO(manufactured by Nisshin Chemical Industry Co., Ltd.), 100OW, DX80,DX81, DX82, DX83 and 100FD (manufactured by Electro Chemical IndustryCo., Ltd.), MR-104, MR-105, MR-110, MR-100, MR-555 and 400X-110A(manufactured by Nippon Zeon Co., Ltd.) as vinyl chloride copolymers;Nippollan N2301, N2302 and N2304 (manufactured by Nippon PolyurethaneCo., Ltd.), Pandex T-5105, T-R3080, T-5201, Burnock D-400, D-210-80,Crisvon 6109 and 7209 (manufactured by Dainippon Chemicals and Ink.),Vylon UR8200, UR8300, UR8700, RV530 and RV280 (manufactured by ToyoboCo., Ltd.), polycarbonate polyurethane, Daipheramine 4020, 5020, 5100,5300, 9020, 9022 and 7020 (manufactured by Dainichi Seika K.K.),polyurethane, MX5004 (manufactured by Mitsubishi Kasei Corp.),polyurethane, Sunprene SP-150 (manufactured by Sanyo Chemical IndustriesCo. Ltd.), polyurethane, Salan F310 and F210 (manufactured by AsahiChemical Industry Co., Ltd.) as polyurethane resins, etc.

The amount of the binder for use in the nonmagnetic layer and themagnetic layer according to the present invention is from 5 to 50% byweight, preferably from 10 to 30% by weight, based on the weight of thenonmagnetic powder or the magnetic powder. When vinyl chloride resinsare used, the amount thereof is from 5 to 30% by weight, whenpolyurethane resins are used, the amount of the polyurethane resin isfrom 2 to 20% by weight, and it is preferred polyisocyanate is used inan amount of from 2 to 20% by weight in combination with these resins.However, for instance, when head corrosion is caused by a slight amountof chlorine due to dechlorination, it is possible to use polyurethanealone or a combination of polyurethane and isocyanate alone.

The magnetic recording medium according to the present invention maycomprise two or more layers. Accordingly, the amount of the binder, theamounts of vinyl chloride resins, polyurethane resins, polyisocyanate orother resins contained in the binder, the molecular weight of each resinconstituting the magnetic layer, the amount of polar groups, or theabove-described physical properties of resins can of course be varied inthe nonmagnetic layer and the magnetic layer, according to necessity.These factors should be rather optimized in respective layers.Well-known techniques with respect to multilayer magnetic layers can beused in the present invention. For example, when the amount of thebinder is varied in each layer, it is effective to increase the amountof the binder contained in the magnetic layer to reduce scratches on thesurface of the magnetic layer. For improving the head touch against thehead, it is effective to increase the amount of the binder in thenonmagnetic layer to impart flexibility.

Examples of the polyisocyanates which can be used in the presentinvention include isocyanates, e.g., tolylenediisocyanate,4,4′-diphenylmethanediisocyanate, hexamethylenediisocyanate,xylylenediisocyanate, naphthylene-1,5-diisocyanate,o-toluidinediisocyanate, isophoronediisocyanate, andtriphenylmethanetriisocyanate; reaction products of these isocyanateswith polyalcohols; and polyisocyanates formed by condensation reactionof isocyanates. These polyisocyanates are commercially available underthe trade names of Coronate L, Coronate HL, Coronate 2030, Coronate2031, Millionate MR and Millionate MTL (manufactured by NipponPolyurethane Co., Ltd.), Takenate D-102, Takenate D-110N, Takenate D-200and Takenate D-202 (manufactured by Takeda Chemical Industries, Ltd.),and Desmodur L, Desmodur IL, Desmodur N and Desmodur HL (manufactured bySumitomo Bayer Co., Ltd.). These polyisocyanates may be used alone or incombinations of two or more thereof, taking advantage of a difference incuring reactivity in each layer.

Carbon Black, Abrasive

Examples of carbon blacks for use in the magnetic layer according to thepresent invention include furnace blacks for rubbers, thermal blacks forrubbers, carbon blacks for coloring, acetylene blacks, etc. Carbonblacks for use in the magnetic layer of the present invention preferablyhave a specific surface area (S_(BET)) of from 5 to 500 m²/g, a DBP oilabsorption of from 10 to 400 ml/100 g, an average particle size of from5 to 300 mμ, pH of from 2 to 10, a water content of from 0.1 to 10%, anda tap density of from 0.1 to 1 g/ml. Specific examples of carbon blacksfor use in the magnetic layer of the present invention includeBLACKPEARLES 2000 (average particle size: 15 nm), 1300 (average particlesize: 13 nm), 1000 (average particle size: 16 nm), 900 (average particlesize: 15 nm), 905, 800 (average particle size: 17 nm), and 700 (averageparticle size: 18 nm), VULCAN XC-72 (average particle size: 30 nm), andSTERLING FT (average particle size: 180 nm) (manufactured by Cabot Co.,Ltd.), #80 (average particle size: 20 nm), #60 (average particle size:49 nm), #55 (average particle size: 68 nm), #50 (average particle size:94 nm), and #35 (average particle size: 94 nm) (manufactured by AsahiCarbon Co., Ltd.), #2400B (average particle size: 15 nm), #2300 (averageparticle size: 15 nm), #900 (average particle size: 16 nm), #1000(average particle size: 18 nm), #30 (average particle size: 30 nm), #40(average particle size: 20 nm), and #10B (average particle size: 84 nm)(manufactured by Mitsubishi Kasei Corp.), CONDUCTEX SC (average particlesize: 17 nm), RAVEN 150 (average particle size: 18 nm), 50 (averageparticle size: 21 nm), 40 (average particle size: 24 nm), and 15(average particle size: 27 nm), RAVEN-MT-P (average particle size: 275nm) and RAVEN-MT-P beads (average particle size: 330 nm) (manufacturedby Columbia Carbon Co., Ltd.), Ketjen Black EC40 (average particle size:30 nm) (manufactured by Akzo Co., Ltd.), and Thermal Black (averageparticle size: 270 nm) (manufactured by Cancarb Co., Ltd.), etc. Carbonblacks for use in the present invention may previously besurface-treated with a dispersant, may be grafted with a resin, or apart of the surface thereof may be graphitized before use. Carbon blacksmay be previously dispersed in a binder before addition to the magneticcoating solution. These carbon blacks may be used alone or incombination. Carbon blacks are preferably used in an amount of from 0.1to 30% by weight based on the amount of the ferromagnetic powder. Carbonblacks can serve various functions such as preventing static charges,reducing a friction coefficient, imparting a light-shielding propertyand improving a film strength. Such functions vary depending upon thekind of carbon blacks to be used. Accordingly, it is of course possiblein the present invention to select and determine the kinds, the amountsand the combinations of the carbon blacks to be added to the uppermagnetic layer and the lower nonmagnetic layer, on the basis of theabove mentioned various properties such as the particle size, the oilabsorption amount, the electroconductivity and the pH value, or theseshould be rather optimized in respective layers. Regarding carbon blacksfor use in the magnetic layer of the present invention, for example, thedisclosure in Handbook of Carbon Blacks (edited by Carbon BlackAssociation of Japan) can be referred to.

As the abrasive usable in the present invention, well-known materialsessentially having a Mohs' hardness of 6 or more may be used alone or incombination. Examples of such abrasives include α-alumina having analpha-conversion rate of 90% or more, β-alumina, silicon carbide,chromium oxide, cerium oxide, α-iron oxide, corundum, artificialdiamond, silicon nitride, silicon carbide, titanium carbide, titaniumoxide, silicon dioxide, and boron nitride. Composites composed of theseabrasives (abrasives obtained by surface-treating with other abrasives)may also be used. Compounds or elements other than the main componentare often contained in these abrasives, but the intended effect can beattained so far as the content of the main component is 90% or more.Abrasives preferably have a particle size of from 0.01 to 2 μm and, inparticular, for improving electro-magnetic characteristics, abrasiveshaving narrow particle size distribution are preferred. For improvingdurability, a plurality of abrasives each having a different particlesize may be combined according to necessity, or a single abrasive havinga broad particle size distribution may be employed so as to attain thesame effect as such a combination. Preferably, abrasives for use in thepresent invention have a tap density of from 0.3 to 2 g/ml, a watercontent of from 0.1 to 5%, a pH value of from 2 to 11 and a specificsurface area (S^(BET)) of from 1 to 30 m²/g. The shape of the abrasivesto be used in the present invention may be any of acicular, sphericaland die-like shapes. Preferably, the abrasive has a shape partly withedges, because a high abrasive property is given. Specific examples ofabrasives for use in the present invention include as examples ofα-alumina, AKP-12 (average particle size: 0.50 μm), AKP-15 (averageparticle size: 0.45 μm), AKP-20 (average-particle size: 0.39 μm), AKP-30(average particle size: 0.23 μm), AKP-50 (average particle size: 0.16μm), HIT-20, HIT-30, HIT-55 (average particle size: 0.20 μm), HIT-60,HIT-70 (average particle size: 0.15 μm), HIT-80 and HIT-100 (averageparticle size: 0.11 μm) (manufactured by Sumitomo Chemical Co., Ltd.),ERC-DBM (average particle size: 0.22 μm), HP-DBM (average particle size:0.22 μm), and HPS-DBM (average particle size: 0.19 μm) (manufactured byReynolds International Inc.), WA10000 (average particle size: 0.29 μm)(manufactured by Fujimi Kenma K.K.), UB20 (average particle size: 0.13μm) (manufactured by Uemura Kogyo K.K.), as examples of chromium oxide,G-5 (average particle size: 0.32 μm), Kromex U2 (average particle size:0.18 μm), and Kromex U1 (average particle size: 0.17 μm) (manufacturedby Nippon Chemical Industrial Co., Ltd.), as examples of α-iron oxide,TF100 (average particle size: 0.14 μm) and TF140 (average particle size:0.17 μm) (manufactured by Toda Kogyo Co., Ltd.), as examples of siliconcarbide, β-Random Ultrafine (average particle size: 0.16 μm)(manufactured by Ibiden Co., Ltd.), and as examples of silicon dioxide,B-3 (average particle size: 0.17 μm) (manufactured by Showa Mining Co.,Ltd.). These abrasives may also be added to a nonmagnetic layer, ifnecessary. By incorporating abrasives into a nonmagnetic layer, it ispossible to control the surface shape or prevent abrasives fromprotruding. Particle sizes and amounts of abrasives to be added to amagnetic layer and a nonmagnetic layer should be selected independentlyat optimal values.

In the case of high capacity floppy discs of rotation rate of 1,800 rpmor more, in particular 3,000 rpm or more, it is preferred to use diamondfine particles as an abrasive.

Diamond fine particles which can be used in the present invention areessential to have an average particle size of from 0.10 to 1.0 μm,preferably from 0.10 to 0.8 μm. When the average particle size is lessthan 0.10 μm, the effect of improving durability is liable to lower ascompared to the addition amount, while when it is larger than 1.0 μm,noise is liable to increase even though durability is improved, which isnot suitable for achieving the object of the present invention.

In the present invention, the maximum size of each diamond particle istaken as a particle size, and the average value of determined values of500 particles by random sampling by means of an electron microscope istaken as an average particle size.

The addition amount of a diamond particle in the present invention isfrom 0.01 to 5% by weight, preferably from 0.03 to 3.00% by weight,based on the weight of the ferromagnetic powder. If the addition amountis less than 0.01% by weight, durability is obtained with difficulty andif it exceeds 5% by weight, the effect of noise reduction by means ofthe addition of a diamond particle is reduced.

The addition amount and-the average particle size of a diamond fineparticle are regulated within the above ranges from the viewpoint ofnoise and durability, but the addition amount thereof is preferably assmall as possible in view of noise. It is preferred in the magneticrecording medium of the present invention to appropriately select theamount and the average particle size of a diamond fine particle suitablefor magnetic recording devices from the above ranges.

Further, with respect to the particle size distribution of a diamondparticle, it is preferred that the number of particles having theparticle size of 200% or more of the average particle size accounts for5% or less of the entire number of diamond particles, and the number ofparticles having the particle size of 50% or less of the averageparticle size accounts for 20% or less of the entire number of diamondparticles. The maximum value of the particle size of the diamond fineparticle for use in the present invention is about 3.00 μm, preferablyabout 2.00 μm, and the minimum value is about 0.01 μm, preferably about0.02 μm.

Particle size distribution is found by counting numbers of respectivesizes based on the average particle size at the time of particle sizedetermination.

Particle size distribution of a diamond fine particle also influencesdurability and noise of the magnetic medium. If the particle sizedistribution is broader than the above-described range, the effectcorresponding to the average particle size set up in the presentinvention deviates as described above, i.e., if many particles have toolarge particle sizes, noise is increased and the head is scratched.While when there exist many particles having too small particle sizes,abrasive effect is insufficient. Further, a diamond fine particle havingextremely narrow particle size distribution is expensive, therefore, theabove-described range is economically advantageous as well.

Diamond fine particles can be used in combination with conventionallyused abrasives, e.g., an alumina abrasive, in the present invention.Better effects on durability and SN ratio are obtained when a smallamount of a diamond fine particle alone is used but, for economicalreason, etc., an alumina abrasive can be used in combination with adiamond fine particle in an amount of preferably from 1 to 30% byweight, more preferably from 3 to 25% by weight, based on the magneticpowder. In this case, addition amount of abrasives can be considerablyreduced due to the addition of a diamond fine particle as compared withthe amount necessary to ensure durability with alumina alone, which ispreferred in view of the security of durability and the reduction ofnoise.

Producing methods of a diamond particle of a micrometer size include (1)a static high pressure method, (2) an explosion method, and (3) a vaporphase method. In a static high pressure method (1), a crystal having aparticle size of several 10 μm or more is prepared in the first place,and the resulting crystal is pulverized to obtain a diamond fineparticle of a sub-micrometer size. In an explosion method (2), extrahigh pressure is generated by shock wave by means of the explosion of anexplosive, and a black smoke is converted to a diamond by making use ofthe generated extra high pressure. The diamond produced by this methodis a polycrystalline diamond which is said to have a primary particle ofsomething between 20 Å and 50 Å. In a vapor phase method (3), a gaseouscompound containing a carbon such as a hydrocarbon is charged into aclosed container under the condition of normal pressure or less with ahydrogen gas, a high temperature zone is formed therein by plasma, etc.,and the starting compound is decomposed to precipitate a diamond on asubstrate, e.g., Si or Mo.

Specific examples of diamond fine particles include LS600F, LS600T,LS600F coated products (coated products coated with 30% or 56% nickel),LS-NPM and BN2600 (manufactured by LANDS SUPERABRASIVES, CO.), which arepreferred as diamond fine particles with arbitrary particle sizes offrom 0 to 100 μm are available. Besides the above, IRM 0-1/4 (averageparticle size: 0.12 μm), and IRM 0-1 (average particle size: 0.60 μm)(manufactured by Tomei Diamond Industrial Co., Ltd.) can be used.

Additive

As additives which can be used in the magnetic layer and the nonmagneticlayer of the present invention, those having a lubrication effect, anantistatic effect, a dispersing effect and a plasticizing effect may beused, and by the combined use of additives, comprehensive improvement ofcapacities can be contrived. As additives having a lubricating effect,lubricants giving remarkable action on adhesion caused by the frictionof surfaces of materials with each other are used. Lubricants areclassified into two types. Lubricants used for a magnetic recordingmedium cannot be judged completely whether they show fluid lubricationor boundary lubrication, but according to general concepts they areclassified into higher fatty acid esters, liquid paraffins and siliconderivatives showing fluid lubrication, and long chain fatty acids,fluorine surfactants and fluorine-containing high polymers showingboundary lubrication. In a coating type magnetic recording medium,lubricants exist in a state dispersed in a binder or in a state partlyadsorbed onto the surface of a ferromagnetic powder, and they migrate tothe surface of a magnetic layer. The speed of migration depends onwhether the compatibility of the binder and the lubricant is good orbad. The speed of migration is slow when the compatibility of the binderand the lubricant is good and the migration speed is fast when thecompatibility is bad. As one idea as to good or bad of thecompatibility, there is a means of comparison of dissolution parametersof both. A nonpolar lubricant is effective for fluid lubrication and apolar lubricant is effective for boundary lubrication. In the presentinvention, at least three in total of these higher fatty acid estershowing fluid lubrication and long chain fatty acid showing boundarylubrication having respectively different characteristics are preferablyused in combination to obtain high capacity, high density and highdurability. Solid lubricants can also be used in combination with these.

Examples of solid lubricants which can be used in the present inventioninclude molybdenum disulfide, tungsten graphite disulfide, boronnitride, and graphite fluoride. Examples of long chain fatty acidsshowing boundary lubrication include monobasic fatty acids having from10 to 24 carbon atoms (which may contain an unsaturated bond or whichmay be branched) and metal salts thereof (e.g., with Li, Na, K or Cu).Examples of fluorine surfactants and fluorine-containing high polymersinclude fluorine-containing silicons, fluorine-containing alcohols,fluorine-containing esters, fluorine-containing alkyl sulfates andalkali metal salts thereof. Examples of higher fatty acid esters showingfluid lubrication include mono-fatty acid esters, di-fatty acid estersor tri-fatty acid esters composed of a monobasic fatty acid having from10 to 24 carbon atoms (which may contain an unsaturated bond or may bebranched) and any one of mono-, di-, tri-, tetra-, penta- andhexa-alcohols having from 2 to 12 carbon atoms (which may contain anunsaturated bond or may be branched), and fatty acid esters of monoalkylethers of alkylene oxide polymers. In addition to the above, examplesfurther include liquid paraffins, and as silicon derivatives, siliconeoils such as dialkylpolysiloxane (the alkyl has from 1 to 5 carbonatoms), dialkoxypolysiloxane (the alkoxy has from 1 to 4 carbon atoms),monoalkylmonoalkoxypolysiloxane (the alkyl has from 1 to 5 carbon atomsand the alkoxy has from 1 to 4 carbon atoms), phenylpolysiloxane, andfluoroalkylpolysiloxane (the alkyl has from 1 to 5 carbon atoms),silicons having a polar group, fatty acid-modified silicons andfluorine-containing silicons.

Examples of other lubricants which can be used in the present inventioninclude alcohols such as mono-, di-, tri-, tetra-, penta- orhexa-alcohols having from 12 to 22 carbon atoms (which may contain anunsaturated bond or may be branched), alkoxy alcohols having from 12 to22 carbon atoms, and fluorine-containing alcohols, polyethylene waxes,polyolefins such as polypropylenes, ethylene glycols, polyglycols suchas polyethylene oxide waxes, alkyl phosphates and alkali metal saltsthereof, alkyl sulfates and alkali metal salts thereof, polyphenylethers, fatty acid amides having from 8 to 22 carbon atoms, andaliphatic amines having from 8 to 22 carbon atoms.

Examples of additives having an antistatic effect, a dispersing effectand a plasticizing effect include phenylphosphonic acids, specificallyPPA (manufactured by Nissan Chemical Industries, Ltd.), etc.,α-naphthylphosphoric acids, phenylphosphoric acids, diphenylphosphoricacids, p-ethylbenzenephosphonic acids, phenylphosphinic acids,aminoquinones, various kinds of silane coupling agents, titaniumcoupling agents, fluorine-containing alkyl sulfates and alkali metalsalts thereof.

Lubricants particularly preferably used in the present invention arefatty acids and fatty acid esters, in addition, other differentlubricants and additives can be used in combination with them as well.Specific examples thereof are exemplified below. As fatty acid, examplesof saturated fatty acids include caprylic acid (C₇H₁₅COOH, meltingpoint: 16° C.), pelargonic acid (C₈H₁₇COOH, melting point: 15° C.),capric acid (C₉H₁₉COOH, melting point: 31.5° C.), undecylic acid(C₁₀H₂₁COOH, melting point: 28.6° C.), lauric acid (C₁₁H₂₃COOH, meltingpoint: 44° C.), specifically NAA-122 (manufactured by Nippon Oils andFats Co., Ltd.), tridecylic acid (C₁₂H₂₅COOH, melting point: 45.5° C.),myristic acid (C₁₃H₂₇COOH, melting point: 58° C.), specifically NAA-142(manufactured by Nippon Oils and Fats Co., Ltd.), pentadecylic acid(C₁₄H₂₉COOH, melting point: 53 to 54° C.), palmitic acid (C₁₅H₃₁COOH,melting point: 63 to 64° C.), specifically NAA-160 (manufactured byNippon Oils and Fats Co., Ltd.), heptadecylic acid (C₁₆H₃₃COOH, meltingpoint: 60 to 61° C.), stearic acid (C₁₇H₃₅COOH, melting point: 71.5 to72° C.), specifically NAA-173K (manufactured by Nippon Oils and FatsCo., Ltd.), nonadecanoic acid (C₁₈H₃₇COOH, melting point: 68.7° C.),arachic acid (C₁₉H₃₉COOH, melting point: 77° C.), and behenic acid(C₂₁H₄₃COOH, melting point: 81 to 82° C.). Examples of unsaturated fattyacids include oleic acid (C₁₇H₃₃COOH(cis), melting point: 16° C.),specifically oleic acid manufactured by Kanto Kagaku Co., Ltd., elaidicacid (C₁₇H₃₃COOH(trans), melting point: 44 to 45° C.), specificallyelaidic acid manufactured by Wako Pure Chemical Industries Ltd.,cetoleic acid (C₂₁H₄₁COOH, melting point: 33.7° C.), erucic acid(C₂₁H₄₁COOH, melting point: 33.4 to 34° C.), specifically erucic acidmanufactured by Nippon Oils and Fats Co., Ltd., brassidic acid(C₂₁H₄₁COOH(trans), melting point: 61.5° C.), linoleic acid (C₁₇H₃₁COOH,boiling point: 228° C. (14 mm)), and linolenic acid (C₁₇H₂₉COOH, boilingpoint: 197° C. (4 mm)). Examples of branched saturated fatty acidsinclude isostearic acid (CH₃CH(CH₃)(CH₂)₁₄COOH, melting point: 67.6 to68.1° C.).

Examples of esters are described below. Examples of laurates includeisocetyl laurate (C₁₁H₂₃COOCH₂CH(C₆H₁₃)C₈H₁₇), oleyl laurate(C₁₁H₂₃COOC₁₈H₃₅), and stearyl laurate (C₁₁H₂₃COOC₁₈H₃₇); examples ofmyristates include isopropyl myristate (C₁₃H₂₇COOCH(CH₃)₂), specificallyEnujerubu IPM (manufactured by Shin-Nihon Rika Co., Ltd.), butylmyristate (C₁₃H₂₇COOC₄H₉), isobutyl myristate (C₁₃H₂₇COO-iso-C₄H₉),specifically Enujerubu IBM (manufactured by Shin-Nihon Rika Co., Ltd.),heptyl myristate (C₁₃H₂₇COOC₇H₁₅), octyl myristate (C₁₃H₂₇COOC₈H₁₇),isooctyl myristate (C₁₃H₂₇COOCH₂CH(C₂H₅)C₄H₉), and isocetyl myristate(C₁₃H₂₇COOCH₂CH(C₆H₁₃)C₈H₁₇).

Examples of palmitates include octyl palmitate (C₁₅H₃₁COOC₈H₁₇), decylpalmitate (C₁₅H₃₁COOC₁₀H₂₁), isooctyl palmitate(C₁₅H₃₁COOCH₂CH(C₂H₅)C₄H₂₁), isocetyl palmitate(C₁₅H₃₁COOCH₂CH(C₆H₁₃)C₈H₁₇), 2-octyldodecyl palmitate(C₁₅H₃₁COOCH₂CH(C₈H₁₇)C₁₂H₂₅), 2-hexyldodecyl palmitate(C₁₅H₃₁COOCH₂CH(C₆H₁₃)C₁₂H₂₅), and oleyl palmitate (C₁₅H₃₁COOC₁₈H₃₅).

Examples of stearates include propyl stearate (C₁₇H₃₅COOC₃H₇), isopropylstearate (C₁₇H₃₅COOCH(CH₃)₂), butyl stearate (C₁₇H₃₅COOC₄H₉),specifically butyl stearate manufactured by Nippon Oils and Fats Co.,Ltd., sec-butyl stearate (C₁₇H₃₅COOCH(CH₃)C₂H₅), tert-butyl stearate(C₁₇H₃₅COOC(CH₃)₃), amyl stearate (C₁₇H₃₅COOC₅H₁₁), isoamyl stearate(C₁₇H₃₅COOCH₂CH₂CH(CH₃)₂), hexyl stearate (C₁₇H₃₅COOC₆H₁₃), heptylstearate (C₁₇H₃₅COOC₇H₁₅), specifically MYB-185 (manufactured byMatsumoto Yushi Co., Ltd.), octyl stearate (C₁₇H₃₅COOC₈H₁₇),specifically N-octyl stearate manufactured by Nippon Oils and Fats Co.,Ltd., isooctyl stearate (C₁₇H₃₅COO-iso-C₈H₁₇), specifically FAL-123(manufactured by Takemoto Yushi Co., Ltd.), decyl stearate (C₁₇H₃₅COOC₁₀_(H) ₂₁), isodecyl stearate (C₁₇H₃₅COO-iso-C₁₀H₂₁), dodecyl stearate(C₁₇H₃₅COOC₁₂H₂₅), isotridecyl stearate (C₁₇H₃₅COO-iso-C₁₃H₂₇),2-ethylhexyl stearate (C₁₇H₃₅COOCH₂CH—(C₂H₅)C₄H₉), isohexadecyl stearateor isocetyl stearate (C₁₇H₃₅COO-iso-C₁₆H₃₃), specifically Enujerubu HDS(manufactured by Shin-Nihon Rika Co., Ltd.), isostearyl stearate(C₁₇H₃₅COO-iso-C₁₈H₃₇), and oleyl stearate (C₁₇H₃₅COOC₁₈H₃₇).

Examples of behenates include isotetracosyl behenate(C₂₁H₄₃COOCH₂CH(C₆H₁₃)C₁₂H₂₅), specifically Enujerubu DTB (manufacturedby Shin-Nihon Rika Co., Ltd.).

Examples of glycol type esters include those disclosed in JP-A-59-227030and JP-A-59-65931, e.g., butoxyethyl stearate (C₁₇H₃₅COOCH₂CH₂OC₄H₈),butoxyethyl oleate (C₁₇H₃₃COOCH₂CH₂OC₄H₉), diethylene glycol monobutylether stearate or butoxyethoxyethyl stearate (C₁₇H₃₅COO(CH₂CH₂O)₂—C₄H₉),tetraethylene glycol monobutyl ether stearate (C₁₇H₃₅COO(CH₂CH₂O)₄C₄H₉),diethylene glycol monophenyl ether stearate (C₁₇H₃₅COO(CH₂CH₂O)₂C₆H₆),and diethylene glycol mono-2-ethylhexyl ether stearate(C₁₇H₃₅COO(CH₂CH₂O)₂CH₂CH(C₂H₅)C₄H₉).

Examples of isostearates include isocetyl isostearate(iso-C₁₇H₃₅COOCH₂CH(C₆H₁₃)C₈H₁₇), specifically I.C.I.S. (manufactured byHigher Alcohol Co., Ltd.), oleyl isostearate (iso-C₁₇H₃₅COOC₁₈H₃₅),stearyl isostearate (iso-C₁₇H₃₅COOC₁₈H₃₇), isostearyl isostearate(iso-C₁₇H₃₅COO-iso-C₁₈H₃₇) and eicosenyl isostearate(iso-C₁₇H₃₅COOC₂₂H₄₃).

Examples of oleates include butyl oleate (C₁₇H₃₃COO—C₄H₉), specificallyEnujerubu BO (manufactured by Shin-Nihon Rika Co., Ltd.), oleyl oleate(C₁₇H₃₃COOC₁₈H₃₅), and ethylene glycol dioleyl(C₁₇H₃₃COOCH₂CH₂OCOC₁₇H₃₃).

Examples of erucic acid ester include oleyl erucate (C₂₁H₄₁COOC₁₈H₃₅).

Examples of diesters include dioleyl maleate (C₁₈H₃₅OCOCH═CHCOOC₁₈H₃₅),neopentyl glycol didecanoate (C₁₀H₂₁COOCH₂C(CH₃)₂CH₂OCOC₁₀H₂₁), ethyleneglycol dilaurate (C₁₁H₂₃COOCH₂CH₂OCOC₁₁H₂₃), ethylene glycol dioleyl(C₁₇H₃₃COOCH₂CH₂OCOC₁₇H₃₃), 1,4-butanediol distearate(C₁₇H₃₅COO(CH₂)₄OCOC₁₇H₃₅), 1,4-butanediol dibehenate(C₂₁H₄₃COO(CH₂)₄OCOC₂₁H₄₃), 1,10-decanediol dioleyl(C₁₇H₃₃COO(CH₂)₁₀OCOC₁₇H₃₃), and 2-butene-1,4-diol cetoleyl(C₂₁H₄₁COOCH₂CH═CHCH₂OCOC₂₁H₄₁)

Examples of triesters include caprylic acid triglyceride(C₇H₁₅COOCH₂CH(OCOC₇H₁₅)CH₂OCOC₇H₁₅.

In addition to the above-described fatty acid esters and fatty acids,examples of additives which can be used include alcohols such as oleylalcohol (C₁₈H₃₅OH), stearyl alcohol (C₁₈H₃₇OH), and lauryl alcohol(C₁₂H₂₅OH).

Examples of fatty acid amides include lauric acid amide (C₁₁H₂₃CONH₂),specifically lauric acid amide manufactured by Tokyo Kasei Co., Ltd.,myristic acid amide (C₁₃H₂₇CONH₂), palmitic acid amide (C₁₅H₃₁CONH₂),oleic acid amide (cis-C₈H₁₇CH═CH(CH₂)₇CONH₂), specifically Armoslip CP-P(manufactured by Lion Akzo Co., Ltd.), erucic acid amide(cis-C₈H₁₇CH═CH(CH₂)₁₁CONH₂), specifically Armoslip E (manufactured byLion Akzo Co., Ltd.), and stearic acid amide (C₁₇H₃₅CONH₂), specificallyArmide HT (manufactured by Lion Akzo Co., Ltd.).

Examples of silicone compounds include TAV-3630, TA-3 and KF-69(manufactured by Shin-Etsu Chemical Co., Ltd.).

Additionally, examples of other additives which may be used includenonionic surfactants such as alkylene oxides, glycerols, glycidols andalkylphenol-ethylene oxide adducts; cationic surfactants such as cyclicamines, ester amides, quaternary ammonium salts, hydantoin derivatives,heterocyclic compounds, phosphoniums and sulfoniums; anionic surfactantscontaining an acidic group such as carboxylic acid, sulfonic acid,phosphoric acid, sulfate groups or phosphate groups; and amphotericsurfactants such as amino acids, aminosulfonic acids, sulfates orphosphates of amino alcohols, and alkylbetains. The details of thesesurfactants are described in Handbook of Surfactants (published bySangyo Tosho Co., Ltd.). These lubricants and antistatic agents may notalways be 100% pure and may contain impurities such as isomers,non-reacted materials, byproducts, decomposed products and oxides, inaddition to the main component. However, the content of such impuritiesis preferably 30% or less, more preferably 10% or less.

As described in Example 35 below, preferred results of the presentinvention can be obtained when a monoester and a diester is used incombination as a fatty acid ester. The details are described below.

That is, the magnetic recording medium of the present invention is ahigh density and high capacity recording medium comprising ahyper-smooth magnetic layer and capable of obtaining stable runningdurability at initial stage of running and after running. Monoesters anddiesters are conventionally used as a lubricant. The present inventorshave earnestly examined characteristics of these lubricants aiming atester groups. As a result of minute examination of behaviors of estergroups in the lower nonmagnetic layer and the magnetic layer, it hasbeen found that as the monoester lubricant has one ester group, which isa polar group, in the molecule, the affinity with a binder is not sohigh, does not remain in the layer and is liable to come out on thesurface of the magnetic layer. On the other hand, as the diesterlubricant has two ester groups, which are polar groups, in the molecule,the affinity with a binder is high, is liable to remain in the layer andis reluctant to come out on the surface of the magnetic layer.Accordingly, it can be presumed that remarkably good running durabilitycan be ensured by contribution of the monoester lubricant at the initialstage of running and by contribution of the diester lubricant afterrunning. Further, the diester lubricant is excellent in low temperaturedurability and the monoester lubricant is excellent in high temperaturedurability. Therefore, when the diester lubricant and the monoesterlubricant are used in combination, markedly excellent running durabilityof from low temperature to high temperature can be obtained. Theseeffects are not merely obtained by the effect of the monoester lubricantplus the effect of the diester lubricant and it is thought to besynergistic effect of two lubricants.

A diester lubricant for use in the present invention is preferablyrepresented by formula (1):

R1—COO—R2—OCO—R3  (1)

wherein R2 represents —(CH₂)_(n)—, a divalent group derived from—(CH₂)_(n)— which may contain an unsaturated bond (wherein n representsan integer of from 1 to 12), —[CH₂CH(CH₃)]—, or —[CH₂C(CH₃)₂CH₂]—; R1and R3, which may be the same or different, each represents achain-like, saturated or unsaturated hydrocarbon group having from 12 to30 carbon atoms.

Herein, “chain-like” of the chain-like hydrocarbon group may be straightchain or branched chain, but it is preferred that both R1 and R3 arestraight and unsaturated, and particularly preferably R1 and R3 have thesame structure. The unsaturated bond may be a double bond or a triplebond but a double bond is preferred and may be one, two or three. Thedouble bond may be either cis or trans.

Carbon atoms of R1 and R3 are respectively from 12 to 30, preferablyfrom 14 to 26, and more preferably from 14 to 20. If carbon atoms areless than 12, the lubricant becomes highly volatile and volatilizes fromthe surface of the magnetic layer during running, which sometimes leadsto running stopping. While when carbon atoms are more than 30, as themobility of the molecule becomes low, it is difficult for the lubricantto bleed out on the surface of the magnetic layer, which sometimes leadsto durability failure.

To make the C/Fe peak ratio from 5 to 120, which is described later,conditions of R1 and R3 are preferably as follows. That is, R1 and R3are alkyl or alkenyl groups, which may be straight or branched butpreferably these groups are groups containing unsaturated bonds whichcan be represented by C═C, and more preferably both groups have the samestructure. R1 and R3 have carbon atoms of from 5 to 21, preferably from7 to 17, and more preferably from 9 to 13. Too short carbon chainlengths of R1 and R3 are not preferred. If carbon chain length is tooshort, the lubricant becomes liable to volatilize, and if the lubricantis liable to volatilize, the lubricant volatilizes and the amount of thelubricant on the surface of the magnetic layer is reduced when thetemperature of the magnetic layer becomes high by the frictional heatgenerated between the magnetic layer and the head. As a result,durability lowers. If carbon chain length is too long, the viscosityincreases and the fluid lubrication performance lowers, as a result, thedurability might be disadvantageously reduced.

R2 is preferably a straight chain divalent alcohol residue having OHgroups on both terminals; and n is preferably from 3 to 12. If n issmall, repeating running durability is deteriorated and, if too large,the viscosity increases and is hard to use as well as durability isliable to fail. Specifically, residues of ethylene glycol, neopentylglycol, propanediol, propylene glycol and butanediol are preferablyused.

The compound represented by formula (1) of the present invention is adiester of a diol represented by HO—R2—OH and an unsaturated fatty acidrepresented by R1—COOH or R3—COOH.

Examples of unsaturated fatty acids represented by R1—COOH or R3—COOHinclude straight chain unsaturated fatty acids, e.g., 4-dodecenoic acid,5-dodecenoic acid, 11-dodecenoic acid, cis-9-tridecenoic acid,myristoleic acid, 5-myristoleic acid, 6-pentadecenoic acid,7-palmitoleic acid, cis-9-palmitoleic acid, 7-heptadecenoic acid, oleicacid, elaidic acid, cis-6-octadecenoic acid, trans-11-octadecenoic acid,cis-11-eicosenoic acid, cis-13-docosenoic acid, 15-tetracosenoic acid,17-hexacosenoic acid, cis-9-octadienoic acid, cis-12-octadienoic acid,trans-9-octadienoic acid, trans-12-octadienoic acid,cis-9-octadecatrienoic acid, trans-11-octadecatrienoic acid,trans-13-octadecatrienoic acid, cis-9-octadecatrienoic acid,cis-12-octadecatrienoic acid, cis-15-octadecatrienoic acid, andstearolic acid; and branched unsaturated fatty acids, e.g.,5-methyl-2-tridecenoic acid, 2-methyl-9-octadecenoic acid,2-methyl-2-eicosenoic acid, and 2,2-dimethyl-11-eicosenoic acid.

Examples of diols represented by HO—R2—OH include straight saturatedterminal diols, e.g., ethylene glycol, trimethylene glycol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-pentanediol,1,8-octanediol, 1,9-nonanediol, and 1,10-decanediol; branched saturateddiols, e.g., propylene glycol, 1,2-butanediol, 1,3-butanediol,2,4-pentanediol, 2,2-dimethyl-1,3-propanediol, 2,5-hexanediol,2-ethyl-1,3-hexanediol, 3-methyl-1,6-hexanediol,1-methyl-1,7-pentanediol, 2,6-dimethyl-1,7-pentanediol, and1-methyl-1,8-nonanediol; straight unsaturated diols, e.g.,2-butene-1,4-diol, 2,4-hexadiene-1,6-dienediol, and 3-pentene-1,7-diol;and branched unsaturated diols, e.g., 2-methyl-2-butene-1,4-diol,2,3-dimethyl-2-butene-1,4-diol, and 2,6-dimethyl-3-hexene-1,6-diol.

Of these, particularly preferred compounds according to the presentinvention are straight chain unsaturated fatty acid esters.Specifically, esters of straight chain unsaturated fatty acids, e.g.,myristoleic acid, 5-myristoleic acid, 7-palmitoleic acid,cis-9-palmitoleic acid, oleic acid, elaidic acid, cis-6-octadecenoicacid (petroselinic acid), trans-6-octadecenoic acid (petroseelaidicacid), trans-11-octadecenoic acid (vaccenic acid), cis-11-eicosenoicacid, cis-13-docosenoic acid (erucic acid), cis-9-octadienoic acid,cis-12-octadienoic acid (linoleic acid), etc., and diethylene glycol,trimethylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,7-pentanediol, 1,8-octanediol, 1,9-nonanediol, or 1,10-decanediol;more preferably esters of the above straight chain unsaturated fattyacids and 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,7-pentanediol, 1,8-octanediol, 1,9-nonanediol, or 1,10-decanediol.Specifically, neopentyl glycol didecanoate, ethylene glycol dioleyl, anddiesters shown below can be exemplified. Examples of diesters are asfollows.

L-a1: C₁₇H₃₅COO(CH₂)₄OCOC₁₇H₃₅

L-a2: C₁₁H₂₁COO(CH₂)₄OCOC₁₁H₂₁

L-a3: C₁₇H₃₃COO(CH₂)₂OCOC₁₇H₃₃

L-a4: C₁₁H₂₃COO(CH₂)₄OCOC₁₁H₂₃

L-a5: C₂₇H₅₃COO(CH₂)₄OCOC₂₇H₅₃

L-a6: C₁₁H₂₁COO(CH₂)₄OCOC₁₇H₃₃

L-a7: C₁₇H₃₃COO(CH₂)₁₁OCOC₁₇H₃₃

L-a8: C₁₇H₃₃COOCH₂CH═CHCH₂OCOC₁₇H₃₃

L-a9: C₁₄H₂₇COOCH₂CH═CHCH₂OCOC₁₄H₂₇

L-a10: C₁₇H₃₃COO(CH₂)₈OCOC₁₄H₂₇

Diesters of dicarboxylic acids and chain-like unsaturated alcohols mayalso be used.

Specific examples of dicarboxylic acids include saturated dicarboxylicacids, e.g., malonic acid, succinic acid, glutaric acid, adipic acid,pimelic acid, suberic acid, azelaic acid, sebacic acid, methylmalonicacid, ethylmalonic acid, propylmalonic acid, and butylmalonic acid; andunsaturated dicarboxylic acids, e.g., maleic acid, fumaric acid,glutaconic acid, itaconic acid, and muconic acid.

Specific examples of chain-like unsaturated alcohols includecis-9-octadecen-1-ol (oleyl alcohol), trans-9-octadecen-1-ol (elaidylalcohol), 9,10-octadecedien-1-ol (linoleyl alcohol),9,12,15-octadecetrien-1-ol (linolenyl alcohol),cis-9-trans-11,13-octadecetrien-1-ol (eleostearyl alcohol),2-pentadecen-1-ol, 2-hexadecen-1-ol, 2-heptadecen-1-ol,2-octadecen-1-ol, and 15-hexadecen-1-ol.

Of the above, particularly preferred compounds according to the presentinvention are esters of straight unsaturated alcohols and saturateddicarboxylic acids. Specifically, preferred compounds are diesters of,as the alcohol ingredient, oleyl alcohol, elaidyl alcohol, linoleylalcohol, linolenyl alcohol, or eleostearyl alcohol, and as thedicarboxylic acid ingredient, malonic acid, succinic acid, glutaricacid, adipic acid, methylmalonic acid, ethylmalonic acid, propylmalonicacid, or butylmalonic acid, and more preferred are diesters of malonicacid or succinic acid, with oleyl alcohol, elaidyl alcohol, linoleylalcohol, or linolenyl alcohol.

Preferred examples of diesters for obtaining C/Fe peak ratio, which isdescribed later, of from 5 to 120 include neopentyl glycol dioleate(L-all), ethylene glycol dioleate (L-a3), neopentyl glycol didecanoate(L-a12), and propanediol dimyristate (L-a13). In addition to these, thefollowing compounds can be exemplified.

C₅H₁₁COOCH₂C(CH₃)₂CH₂OCOC₅H₁₁

C₇H₁₅COOCH₂C(CH₃)₂CH₂OCOC₇H₁₅

C₈H₁₉COOCH₂C(CH₃)₂CH₂OCOC₈H₁₉

C₁₁H₂₃COOCH₂C(CH₃)₂CH₂OCOC₁₁H₂₃

C₁₃H₂₇COOCH₂C(CH₃)₂CH₂OCOC₁₃H₂₇

C₁₇H₃₅COOCH₂C(CH₃)₂CH₂OCOC₁₇H₃₅

C₂₁H₄₃COOCH₂C(CH₃)₂CH₂OCOC₂₁H₄₃

C₄H₇COOCH₂C(CH₃)₂CH₂OCOC₄H₇

C₂₂H₄₅COOCH₂C(CH₃)₂CH₂OCOC₂₂H₄₅

C₁₇H₃₅COOCH₂C(CH₃)₂CH₂OCOC₁₃H₂₇

A monoester lubricant for use in the present invention is preferablyrepresented by formula (2) or (3):

R4—COO—(R5—O)_(m)—R6  (2)

R7—COO—R8  (3)

wherein m represents an integer of from 1 to 10; R5 represents—(CH₂)_(n)—, or a divalent group derived from —(CH₂)_(n)— which maycontain an unsaturated bond (wherein n represents an integer of from 1to 10); R4 and R7, which may be the same or different, each represents achain-like, saturated or unsaturated hydrocarbon group having from 12 to26 carbon atoms; and R6 and R8, which may be the same or different, eachrepresents a chain-like or branched, saturated or unsaturatedhydrocarbon group having from 1 to 26 carbon atoms.

Monofatty acid esters comprising a monobasic fatty acid having from 10to 24 carbon atoms (which may contain an unsaturated bond or may bebranched) and a monovalent alcohol having from 2 to 24 carbon atoms(which may contain an unsaturated bond or may be branched) may be used.

Specific examples of monoesters include butyl stearate, octyl stearate,amyl stearate, isooctyl stearate, butyl myristate, octyl myristate,butoxyethyl stearate, butoxydiethyl stearate, 2-ethylhexyl stearate,2-octyldodecyl palmitate, 2-hexyldodecyl palmitate, isohexadecylstearate, oleyl oleate, dodecyl stearate, tridecyl stearate, and oleylerucate.

In addition to the above compounds, as is well known, as disclosed inJP-B-51-39081, monoesters of saturated and unsaturated fatty acids andalcohols, and oleyl oleate as a fatty acid monoester having anunsaturated bond, as disclosed in JP-B-4-4917, can also be used.Specific examples of monoesters are shown below.

L-b1: C₁₇H₃₅COOC₁₇H₃₅

L-b2: C₁₇H₃₅COOC₄H₉

L-b3: C₁₇H₃₅COOCH₂CH₂OC₄H₉

L-b4: C₁₇H₃₅COO(CH₂CH₂O)₂C₄H₉

Ester lubricants which are used in the present invention are added tothe upper magnetic layer in an amount of 1 weight part or more,preferably 3 weight parts or more, and more preferably 5 weight parts ormore, per 100 weight parts of the ferromagnetic metal powder containedin the upper magnetic layer, and to the lower nonmagnetic layer in anamount of 1 weight part or more, preferably 3 weight parts or more, andmore preferably 5 weight parts or more, per 100 weight parts of thenonmagnetic powder contained in the lower layer. Ester lubricants arepreferably added to both of the upper layer and the lower layer. Theupper limit of the addition amount is 20% with each layer. Too much anamount coarsens the magnetic layer surface thereby the magneticcharacteristics lowers, and too small an amount deteriorates thedurability. Diester lubricants and ester lubricants are contained in anamount of from 10 to 30 weight parts, preferably from 12 to 20 weightparts, per 100 weight parts of the ferromagnetic powder contained in themagnetic layer or per 100 weight parts of the nonmagnetic powdercontained in the lower layer. Diester lubricants and ester lubricantsmay be used in admixture. In this case, the proportion of diesterlubricants is preferably 30% or more based on the total amount of thediester and ester lubricants.

Further, the present invention comprises a magnetic recording mediumwhich comprises a support having thereon a substantially nonmagneticlower layer and a magnetic layer comprising a ferromagnetic metal powderdispersed in a binder provided on the lower layer, wherein the magneticlayer contains from 10 to 30 weight parts of a fatty acid ester per 100weight parts of the ferromagnetic metal powder and/or the nonmagneticlower layer contains from 10 to 30 weight parts of a fatty acid esterper 100 weight parts of the nonmagnetic powder contained in the lowerlayer, the surface of the magnetic layer has a C/Fe peak ratio of from 5to 120 when the surface is measured by the Auger electron spectroscopy,and the magnetic recording medium is a disc-like medium. Although theamount of the ester or diester lubricant contained in the magnetic layerand the lower layer is almost the same with that of conventional floppydiscs, extremely high durability, high hardness of the magnetic layersurface and high scratch resistance can be ensured by the constructionof the present invention by suppressing the amount of the lubricantexisting on the magnetic layer surface within a low value. It has beenfound that the magnetic recording medium according to the presentinvention has achieved conspicuous durability above all in a highrotation recording system of 1,800 rpm or more (e.g., ZIP), inparticular, 3,000 rpm or more (e.g., HiFD®).

The C/Fe peak ratio of the magnetic layer surface by the Auger electronspectroscopy in the present invention is an index which shows theexisting amount of the lubricant on the magnetic layer surface.

This is a method making use of a principle of determination of theamount of the element from the amount of Auger electron beam by applyingelectron beam to the sample and deciding the kind of element from thekinetic energy of Auger electron coming from the sample.

When the magnetic layer surface is spectrally analyzed by the Augerelectron spectroscopy, the peak of iron atom coming from the magneticpowder and the peak of carbon atom coming from the binder and thelubricant appear. However, the carbon atom peak mostly originates in thelubricant. The basis for this is the fact that when the magnetic layersurface of the magnetic disc of the present invention is determined bythe Auger electron spectroscopy with the lubricant of the presentinvention being removed by hexane treatment, Fe peak appears stronglybut C peak to which the binder contributes to is weak; on the contrary,when the determination is conducted without subjecting to hexanetreatment, C peak appears strongly. That is, when the magnetic layersurface is spectrally analyzed by the Auger electron spectroscopy, thepeak of iron atom coming from the magnetic powder and the peak of carbonatom coming from the binder and the lubricant appear, however, thecarbon atom peak can be considered to mostly originate in the lubricantaccording to the present invention.

In the present invention, determination of the C/Fe peak by the Augerelectron spectroscopy is conducted as follows.

Apparatus: PHI-660 type manufactured by Φ Co.

Conditions of determination:

Primary electron beam, accelerating voltage: 3 KV

Electric current of sample: 130 nA

Magnification: 250-fold

Inclination angle: 30°

The value of C/Fe peak is obtained as the C/Fe ratio by integrating thevalues obtained under the above conditions in the region of kineticenergy of 130 eV to 730 eV three times and finding the strengths of KLLpeak of the carbon and LMM peak of the iron as differentials.

The C/Fe peak ratio of the magnetic layer surface of the disc-likemagnetic recording medium according to the present invention determinedby the Auger electron spectroscopy is from 5 to 120. On the contrary,those of conventional floppy discs are 100 or more. From this fact, itcan be seen that the amount of the lubricant present on the magneticlayer surface of the disc-like magnetic recording medium according tothe present invention is markedly small as compared with the amount inconventional floppy discs.

On the other hand, the amount of the lubricants contained in each of themagnetic layer and the lower layer of the disc-like magnetic recordingmedium according to the present invention is from 10 to 30 weight partsrespectively per 100 weight parts of the ferromagnetic powder ornonmagnetic powder. This is almost the same amount as the amountcontained in conventional floppy discs.

Accordingly, although the amount of the lubricant contained in themagnetic layer and the lower layer of the present invention is almostthe same with that of conventional floppy discs, the amount of thelubricant present on the magnetic layer surface is markedly small ascompared with the amount in conventional floppy discs.

Conventional floppy discs have drawbacks such that if the amount of alubricant is increased to improve durability, the amount of thelubricant on the surface increases, as a result, the magnetic layersurface adheres to the magnetic head at still time and the startingtorque becomes large. If the amount of the lubricant is reduced to lowerthe starting torque, friction coefficient increases and durability isdeteriorated. These drawbacks are more conspicuous by high rotationdriving for high density recording.

Although the amount of the ester or diester lubricant contained in themagnetic layer and the lower layer is almost the same with that ofconventional floppy discs, extremely high durability, high hardness ofthe magnetic layer surface and high scratch resistance can be ensured bysuppressing the amount of the lubricant existing on the magnetic layersurface within a low value. Above all, the magnetic recording mediumaccording to the present invention has achieved conspicuous durabilityin a high rotation recording system of 700 rpm or more, more preferably1,800 rpm or more (e.g., ZIP), in particular, 3,000 rpm or more (e.g.,HiFD®).

Moreover, as a large amount of lubricant is contained in the inside ofthe magnetic layer and the lower layer and it comes out on the surfacegradually and exhibits lubricating function, the magnetic recordingmedium of the present invention is excellent in long term storagestability.

To realize the existing mode of the lubricant according to the presentinvention, i.e., a large amount of lubricant is contained in the insideof the magnetic layer and the lower layer and an appropriate amount ispresent on the magnetic layer surface (from 5 to 120 in terms of C/Fevalue obtained mainly from the detected amount of the carbon atom of thelubricant and the iron atom of the magnetic powder by the Auger electronspectroscopy), the following means can be exemplified.

1. The lubricant comprises ester compounds and diester compounds, inparticular, diester compounds having an unsaturated C═C bond and estercompounds have affinity with the binder and the surface of thenonmagnetic powder and preferred. The amount of the lubricant in eachlayer is from 10 to 30 weight parts per 100 weight parts of theferromagnetic powder and the nonmagnetic powder, respectively.

2. It is preferred that the amount of the binder in the lower layer islarger than the amount contained in the upper magnetic layer, i.e., theamount of the binder including the curing agent contained in themagnetic layer is from 10 to 25 weight parts per 100 weight parts of theferromagnetic powder and the amount of the binder contained in the lowerlayer is from 25 to 40 weight parts per 100 weight parts of thenonmagnetic powder.

3. The binder for the lower layer particularly preferably comprises thestructure having a strong polar group such as SO₃Na and the skeletoncontaining many aromatic rings, thereby the affinity of the lubricantwith the lower layer binder increases and much lubricant can be presentin the lower layer stably. If the affinity of the lubricant with thebinder is too high and the binder is completely compatible with thelubricant at the molecular level, the lubricant disadvantageously cannotmigrate to the upper layer.

On the surface of the disc-like recording medium of the presentinvention, ester and diester compounds exist in sufficient amount,although the amount thereof is not more than the amount contained inconventional discs. Therefore, if the temperature increases due to thefrictional heat between the disc and the magnetic head generated by highrotation, the lubricant is difficult to volatilize by virtue of strongintermolecular interaction. Accordingly, stable fluid lubrication can bemaintained without causing breaking of a lubricant film.

In the present invention, the storage stability of the magneticrecording medium at high temperature and high humidity can be improvedwhen the Al/Fe ratio of the ferromagnetic metal powder is from 5 atomic% to 30 atomic %. A diester compound is originally highly hydrophilicand hygroscopic and is susceptible to hydrolysis in nature. Thisproperty is heightened by the catalytic activity of surfaces of magneticpowders, and when stored at high temperature hich humidity, diester isfurther susceptible to hydrolysis. When the Al/Fe ratio of theferromagnetic metal powder is in the range of from 5 atomic % to 30atomic %, the influence is small and insusceptible to decomposition. Asa result, the durability of the disc is hardly reduced andcharacteristics of the disc can be exhibited even after being storedunder high temperature and high humidity conditions.

Lubricants and surfactants for use in the present invention respectivelyhave different physical functions. The kinds, amounts and proportions ofcombination generating synergistic effect of these lubricants should bedetermined optimally in accordance with the purpose. The nonmagneticlayer and the magnetic layer can separately contain different fattyacids each having a different melting point so as to prevent bleedingout of the fatty acids to the surface, or different esters each having adifferent boiling point, a different melting point or a differentpolarity so as to prevent bleeding out of the esters to the surface.Also, the amounts of surfactants are controlled so as to improve thecoating stability, or the amount of the lubricant in the lower layer ismade larger than the amount in the magnetic layer so as to improve thelubricating effect of the surface thereof. Examples are by no meanslimited thereto. In general, the total amount of the lubricants is from0.1 to 50%, preferably from 2 to 25%, based on the amount of themagnetic powder or the nonmagnetic powder.

All or a part of the additives to be used in the present invention maybe added to the magnetic coating solution or the nonmagnetic coatingsolution in any step of the preparation. For example, additives may beblended with a magnetic powder before the kneading step, may be addedduring the step of kneading a magnetic powder, a binder and a solvent,may be added during the dispersing step, may be added after thedispersing step, or may be added immediately before coating. Accordingto the purpose, there is a case of capable of attaining the object bycoating all or a part of the additives simultaneously with orsuccessively after the coating of the magnetic layer. According to thepurpose, lubricants may be coated on the surface of the magnetic layerafter the calendering treatment or after the completion of slitting.

Layer Construction

The thickness of the nonmagnetic support in the magnetic recordingmedium of the present invention is, for example, from 2 to 100 μm,preferably from 2 to 80 μm. Particularly, the thickness of thenonmagnetic support for a computer tape is from 3.0 to 6.5 μm,preferably from 3.0 to 6.0 μm, more preferably from 4.0 to 5.5 μm.

An undercoating layer (or a subbing layer) may be provided between thenonmagnetic flexible support and the nonmagnetic or magnetic layer foradhesion improvement. The thickness of this undercoating layer is from0.01 to 0.5 μm, preferably from 0.02 to 0.5 μm. The nonmagnetic layerand the magnetic layer of the magnetic recording medium according to thepresent invention may be provided on both surface sides of the supportor may be provided on either one surface side. When the nonmagneticlayer and the magnetic layer are provided on only one surface side ofthe support, a back coating layer may be provided on the surface side ofthe support opposite to the side having the nonmagnetic layer andmagnetic layer for the purpose of static charge prevention and curlingcorrection. The thickness of this back coating layer is from 0.1 to 4μm, preferably from 0.3 to 2.0 μm. Well-known undercoating layers andback coating layers can be used for this purpose.

The thickness of the magnetic layer of the magnetic recording medium ofthe present invention can be optimally selected according to thesaturation magnetization amount of the head used, the head gap length,and the recording signal zone, and is generally from 0.05 to 0.25 μm,preferably from 0.05 to 0.20 μm. The magnetic layer may comprise two ormore layers each having different magnetic characteristics andwell-known multilayer magnetic layer structures can be applied to thepresent invention.

The thickness of the lower nonmagnetic layer of the medium according tothe present invention is generally from 0.2 to 5.0 μm, preferably from0.3 to 3.0 μm, and more preferably from 1.0 to 2.5 μm. The lower layerof the recording medium of the present invention exhibits the effect ofthe present invention so long as it is substantially a nonmagnetic layereven if, or intendedly, it contains a small amount of a magnetic powderas an impurity, which is as a matter of course regarded as essentiallythe same construction as in the present invention. The term“substantially a nonmagnetic layer” means that the residual magneticflux density of the lower layer is 100 G or less and the coercive forceof the lower layer is 100 Oe or less, preferably the residual magneticflux density and the coercive force are zero.

Back Coating Layer

In general, a magnetic tape for a computer data recording is decidedlyrequired to have an excellent repeating-running property as comparedwith a video tape and an audio tape. For maintaining such a high runningdurability, it is preferred for the back coating layer to contain acarbon black and an inorganic powder.

Two kinds of carbon blacks respectively having different averageparticle sizes are preferably used in combination. In this case, acombined use of a fine carbon black having an average particle size offrom 10 to 20 mμ and a coarse carbon black having an average particlesize of from 230 to 300 mμ is preferred. In general, by theincorporation of a fine carbon black as above, the surface electricalresistance of the back coating layer and light transmittance can be setup at low values. There are many kinds of magnetic recording apparatusesmaking use of light transmittance of a tape and making it as signals ofoperation, therefore, the addition of fine carbon blacks areparticularly effective in such a case. In addition, a fine carbon blackis in general excellent in retention of a liquid lubricant andcontributes to the reduction of a friction coefficient when a lubricantis used in combination. On the other hand, a coarse carbon black havinga particle size of from 230 to 300 mμ has a function as a solidlubricant and, forms minute protrusions on the surface of a back coatinglayer to reduce contact area and contributes to the reduction of afriction coefficient. However, a coarse carbon black has a drawback suchthat particles are liable to drop out from the back coating layer due tothe tape sliding during severe running leading to the increase of theerror rate.

Specific examples of fine carbon blacks commercially available includeRAVEN 2000B (average particle size: 18 mμ) and RAVEN 1500B (averageparticle size: 17 mμ) (manufactured by Columbia Carbon Co., Ltd.), BP800(average particle size: 17 mμ) (manufactured by Cabot Co., Ltd.),PRINTEX90 (average particle size: 14 mμ), PRINTEX95 (average particlesize: 15 mμ), PRINTEX85 (average particle size: 16 mμ), PRINTEX75(average particle size: 17 mμ) (manufactured by Degussa Co., Ltd.), and#3950 (average particle size: 16 mμ) (manufactured by Mitsubishi KaseiCorp.).

Specific examples of coarse carbon blacks commercially available includeTHERMAL BLACK (average particle size: 270 mμ) (manufactured by CancarbCo., Ltd.) and RAVEN MTP (average particle size: 275 mμ) (manufacturedby Columbia Carbon Co., Ltd.).

When two kinds of carbon blacks respectively having different averageparticle sizes are used in combination in a back coating layer, theproportion of the contents (by weight) of a fine carbon black having aparticle size of from 10 to 20 mμ and a coarse carbon black having aparticle size of from 230 to 300 mμ is preferably the former/the latterof from 98/2 to 75/25, more preferably from 95/5 to 85/15.

The content of the carbon black in a back coating layer (the totalamount when two kinds are used) is generally from 30 to 80 weight parts,preferably from 45 to 65 weight parts, based on 100 weight parts of thebinder.

It is preferred to use two kinds of inorganic powders respectivelyhaving different hardness.

Specifically, a soft inorganic powder having a Mohs' hardness of from 3to 4.5 and a hard inorganic powder having a Mohs' hardness of from 5 to9 are preferably used in combination.

By the addition of a soft inorganic powder having a Mohs' hardness offrom 3 to 4.5, a friction coefficient can be stabilized againstrepeating-running. Moreover, a sliding guide pole is not scratched offin hardness within this range. The average particle size of such a softinorganic powder is preferably from 30 to 50 mμ.

Examples of soft inorganic powders having a Mohs' hardness of from 3 to4.5 include, e.g., calcium sulfate, calcium carbonate, calcium silicate,barium sulfate, magnesium carbonate, zinc carbonate and zinc oxide. Theycan be used alone or in combination of two or more. Of these, calciumcarbonate is particularly preferred.

The content of the soft inorganic powder in a back coating layer ispreferably from 10 to 140 weight parts, more preferably from 35 to 100weight parts, based on 100 weight parts of the carbon black.

By the addition of a hard inorganic powder having a Mohs' hardness offrom 5 to 9, the strength of the back coating layer is increased andrunning durability is improved. When such hard inorganic powders areused together with carbon blacks and the above-described soft inorganicpowders, deterioration due to repeating sliding is reduced and strongback coating layer can be obtained. Appropriate abrasive capability isimparted to the back coating layer by the addition of the hard inorganicpowder and the adhesion of scratched powders to a tape guide pole isreduced. In particular, when the hard inorganic powder is used incombination with a soft inorganic powder (in particular, calciumcarbonate), sliding characteristics against a guide pole having a roughsurface is improved and the stabilization of a friction coefficient ofthe back coating layer can also be brought about.

The average particle size of hard inorganic powders is preferably from80 to 250 mμ, more preferably from 100 to 210 mμ.

Examples of hard inorganic powders having a Mohs' hardness of from 5 to9 include, e.g., α-iron oxide, α-alumina, and chromium oxide (Cr₂O₃).These powders may be used alone or in combination. Of the above, α-ironoxide and α-alumina are preferred. The content of hard inorganic powdersin the back coating layer is generally from 3 to 30 weight parts,preferably from 3 to 20 weight parts, based on 100 weight parts of thecarbon black.

When the above soft inorganic powder and hard inorganic powder are usedin combination in the back coating layer, it is preferred to use themselectively such that the difference of hardness between soft and hardinorganic powders is 2 or more, more preferably 2.5 or more, andparticularly preferably 3 or more.

It is preferred that the above-described two kinds of inorganic powdersrespectively having different hardness and specific average particlesizes and the above-described two kinds of carbon blacks respectivelyhaving different specific average particle sizes are contained in theback coating layer. In particular, in this combination, calciumcarbonate is preferably contained as a soft inorganic powder.

Lubricants may be contained in the back coating layer. Lubricants can bearbitrarily selected from among those which can be used in a magneticlayer or a nonmagnetic layer as described above. The content oflubricants added to the back coating layer is generally from 1 to 5weight parts based on 100 weight parts of the binder.

Support

The support for use in the present invention essentially has a thermalshrinkage factor of 0.5% or less both at 100° C. for 30 minutes and at80° C. for 30 minutes in every direction of in-plane of the support.Moreover, the above-described thermal shrinkage factors of the supportat 100° C. for 30 minutes and at 80° C. for 30 minutes are preferablyalmost equal in every direction of in-plane of the support withdifference of not more than 10%. The support is preferably a nonmagneticsupport. As a nonmagnetic support for use in the present invention,well-known films such as polyesters (e.g., polyethylene terephthalate orpolyethylene naphthalate), polyolefins, cellulose triacetate,polycarbonate, polyamide, polyimide, polyamideimide, polysulfone,polyaramide, aromatic polyamide, or polybenzoxazole can be used. Highlystrong supports such as polyethylene naphthalate or polyamide arepreferably used. If necessary, a lamination type support as disclosed inJP-A-3-224127 can be used to vary the surface roughnesses of themagnetic layer surface and the base surface. The support may bepreviously subjected to surface treatments, such as a corona dischargetreatment, a plasma treatment, an adhesion assisting treatment, a heattreatment, and a dust removing treatment. Aluminum or glass substratecan also be used as a support in the present invention.

For attaining the object of the present invention, it is necessary touse the support having a central plane average surface roughness (SRa)of 8.0 nm or less, preferably 4.0 nm or less, more preferably 2.0 nm orless, measured by a surface roughness meter “TOPO-3D” (a product of WYKOCo., Ltd., U.S.A.) by MIRAU method. It is preferred that the support notonly has a small central plane average surface roughness but also isfree from coarse protrusions (having a height) of 0.5 μm or more.Surface roughness configuration is freely controlled by the size and theamount of fillers added to the support. Examples of such fillers includeacryl-based organic powders, as well as oxides or carbonates of Ca, Si,Ti and Al. It is also preferred to use a Cl-containing compound as acomponent of the support for easy dissolution in an organic solvent. Thesupport for use in the present invention preferably has the maximumheight (SRmax) of 1 μm or less, ten point average roughness (SRz) of 0.5μm or less, central plane peak height (SRp) of 0.5 μm or less, centralplane valley depth (SRv) of 0.5 μm or less, central plane area factor(SSr) of from 10% to 90%, and average wavelength (Sλa) of from 5 μm to300 μm. For obtaining desired electromagnetic characteristics anddurability, surface protrusion distribution of the support can becontrolled arbitrarily by fillers, e.g., the number of protrusionshaving sizes of from 0.01 μm to 1 μm can be controlled each within therange of from 0 to 2,000 per 0.1 mm².

The F-5 value of the support for use in the present invention ispreferably from 49 to 490 MPa (5 to 50 kg/mm²), a thermal shrinkagefactor of the support at 100° C. for 30 minutes is preferably 3% orless, more preferably 1.5% or less, and a thermal shrinkage factor at80° C. for 30 minutes is preferably 1% or less, more preferably 0.5% orless. The support has a breaking strength of from 49 to 980 MPa (5 to100 kg/mm²), an elastic modulus of from 100 to 2,000 kg/mm², atemperature expansion coefficient of from 10⁻⁴ to 10⁻⁸/° C., preferablyfrom 10⁻⁵ to 10⁻⁶/° C., and a humidity expansion coefficient of 10⁻⁴/RH% or less, preferably 10⁻⁵/RH % or less. These thermal characteristics,dimensional characteristics and mechanical strength characteristics arepreferably almost equal in every direction of in-plane of the supportwith difference of not more than 10%.

Producing Method

Processes of preparing the magnetic coating solution for use in themagnetic recording medium of the present invention comprises at least akneading step, a dispersing step and, optionally, blending steps to becarried out before and/or after the kneading and dispersing steps. Anyof these respective steps may be composed of two or more separatestages. Materials such as a magnetic powder, a nonmagnetic powder, abinder, a carbon black, an abrasive, an antistatic agent, a lubricant, asolvent, and the like for use in the present invention may be added atany step at any time. Each material may be added at two or more stepsdividedly. For example, polyurethane can be added dividedly at akneading step, a dispersing step, or a blending step for adjustingviscosity after dispersion. For achieving the object of the presentinvention, the above steps can be performed partly with conventionallywell-known techniques. Powerful kneading machines such as an openkneader, a continuous kneader, a pressure kneader or an extruder arepreferably used in a kneading step. When a kneader is used, all or apart of the binder (preferably 30% or more of the total binders) arekneading-treated in the range of from 15 parts to 500 parts per 100parts of the magnetic powder or nonmagnetic powder together with amagnetic powder or a nonmagnetic powder. Details of these kneading aredisclosed in JP-A-1-106338 and JP-A-1-79274. When dispersing a magneticlayer solution and a nonmagnetic layer solution, glass beads can be usedbut dispersing media having a high specific gravity is preferably usedand zirconia beads, titania beads and steel beads are suitable for thispurpose. Optimal particle size and packing density of these dispersingmedia should be selected. Well-known dispersing apparatuses can be usedin the present invention.

The following methods are preferably used for coating the magneticrecording medium having a multilayer construction of the presentinvention. As the first method, the lower layer is coated by any ofgravure coating, roll coating, blade coating, and extrusion coatingapparatuses, which are ordinarily used in the coating of a magneticcoating solution, and the upper layer is coated while the lower layer isstill wet by means of the support pressing type extrusion coatingapparatus disclosed in JP-B-1-46186, JP-A-60-238179 and JP-A-2-265672.As the second method, the upper layer and the lower layer are coatedalmost simultaneously using the coating head equipped with two slits forfeeding coating solution as disclosed in JP-A-63-88080, JP-A-2-17971 andJP-A-2-265672. And as the third method, the upper layer and the lowerlayer are coated almost simultaneously using the extrusion coatingapparatus equipped with a backup roll as disclosed in JP-A-2-174965. Forpreventing the deterioration of the electromagnetic characteristics ofthe magnetic recording medium due to agglomeration of magnetic powders,it is preferred to impart shear to the coating solution in the coatinghead by the methods as described in JP-A-62-95174 and JP-A-1-236968.With respect to the viscosity of the coating solution, the range of thenumeric values disclosed in JP-A-3-8471 is necessary to be satisfied.For realizing the constitution of the present invention, successivemultilayer coating method in which the magnetic layer is coated on thelower layer after the lower layer has been coated and dried can ofcourse be used without impairing the effect of the present invention.However, for reducing coating defects and improving quality, e.g.,dropout, it is preferred to use the above simultaneous multilayercoating method.

In the case of a magnetic disc, isotropic orienting property can besufficiently obtained in some cases without conducting orientation usingorientating apparatus, but it is preferred to use well-known randomorientation apparatuses, such as disposing cobalt magnets diagonally andalternately or applying an alternating current magnetic field using asolenoid. Isotropic orientation in a ferromagnetic metal fine powder isin general preferably in-plane two dimensional random orientation, butit may be three dimensional random orientation having verticalcomponents. Hexagonal ferrites in general have an inclination for threedimensional random orientation of in-plane and in the vertical directionbut it can be made in-plane two dimensional random orientation.

Further, it is possible to impart isotropic magnetic characteristics inthe circumferential direction by vertical orientation using well-knownmethods, e.g., using different pole and counter position magnets. Inparticular, vertical orientation is preferred when the disc is used inhigh density recording. Circumferential orientation can be conductedusing spin coating.

In the case of a magnetic tape, orientation is conducted in the machinedirection using a cobalt magnet and a solenoid. In orientation, it ispreferred that the drying position of the coated film can be controlledby controlling the temperature and the amount of drying air and coatingrate. Coating rate is preferably from 20 to 1,000 m/min. and thetemperature of drying air is preferably 60° C. or more. Preliminarydrying can be performed appropriately before entering the magnet zone.

Use of heat resisting plastic rolls such as epoxy, polyimide, polyamideand polyimideamide, or metal rolls is effective for calenderingtreatment. Metal rolls are usable for the treatment particularly whenmagnetic layers are coated on both surface sides. Treatment temperatureis preferably 50° C. or more, more preferably 100° C. or more. Linepressure is preferably 200 kg/cm or more, more preferably 300 kg/cm ormore.

Physical Properties

Saturation magnetic flux density of the magnetic layer of the magneticrecording medium according to the present invention is from 2,000 to5,000 G when a ferromagnetic metal powder is used, and from 1,000 to3,000 G when a hexagonal ferrite is used. Coercive force (Hc) and (Hr)are from 1,500 to 5,000 Oe, preferably from 1,700 to 3,000 Oe. Coerciveforce distribution is preferably narrow, and SFD and SFDr are preferably0.6 or less. Squareness ratio is from 0.55 to 0.67, preferably from 0.58to 0.64 in the case of two dimensional random orientation, from 0.45 to0.55 in the case of three dimensional random orientation, and in thecase of vertical orientation, 0.6 or more, preferably 0.7 or more in thevertical direction, and when diamagnetical correction is conducted, 0.7or more, preferably 0.8 or more Orientation ratio of two dimensionalrandom orientation and three dimensional random orientation ispreferably 0.8 or more. In the case of two dimensional randomorientation, squareness ratio, Br, Hc and Hr in the vertical directionare preferably from 0.1 to 0.5 times of those in the in-plane direction.

In the case of a magnetic tape, squareness ratio is 0.7 or more,preferably 0.8 or more.

The friction coefficient of the magnetic recording medium according tothe present invention against a head at temperature of −10° C. to 40° C.and humidity of 0% to 95% is 0.5 or less, preferably 0.3 or less, thesurface inherent resistivity of the magnetic surface is preferably from10⁴ to 10¹² Ω/sq, the charge potential is preferably from −500 V to +500V, the elastic modulus at 0.5% elongation of the magnetic layer ispreferably from 100 to 2,000 kg/mm² in every direction of in-plane, thebreaking strength is preferably from 10 to 70 kg/mm², the elasticmodulus of the magnetic recording medium is preferably from 100 to 1,500kg/mm² in every direction of in-plane, the residual elongation ispreferably 0.5% or less, and the thermal shrinkage factor at everytemperature of 100° C. or less is preferably 1% or less, more preferably0.5% or less, and most preferably 0.1% or less. The glass transitiontemperature of the magnetic layer (the maximum of elastic modulus lossby dynamic visco-elasticity measurement at 110 Hz) is preferably from50° C. to 120° C., and that of the lower nonmagnetic layer is preferablyfrom 0° C. to 100° C. The elastic modulus loss is preferably within therange of from 1×10⁶ to 8×10⁹ dyne/cm², and loss tangent is preferably0.2 or less. If loss tangent is too large, adhesion failure is liable tooccur. These thermal and mechanical characteristics are preferablyalmost equal in every direction of in-plane of the medium withdifference of not more than 10%. The residual amount of the solvent inthe magnetic layer is preferably 100 mg/m² or less, more preferably 10mg/m² or less. The void ratio is preferably 30% by volume or less, morepreferably 20% by volume or less, with both of the nonmagnetic layer andthe magnetic layer. The void ratio is preferably smaller for obtaininghigh output but in some cases a specific value should be preferablysecured depending on purposes. For example, in a disc-like medium whichis repeatedly used, large void ratio contributes to good runningdurability in many cases.

The magnetic layer has a central plane surface roughness (Ra) of 4.0 nmor less, preferably 3.8 nm or less, and more preferably 3.5 nm or less,measured by a surface roughness meter “TOPO-3D” (a product of WYKO Co.,Ltd., U.S.A.) by MIRAU method. The magnetic layer for use in the presentinvention preferably has the maximum height (SRmax) of 0.5 μm or less,ten point average roughness (SRz) of 0.3 μm or less, central plane peakheight (SRp) of 0.3 μm or less, central plane valley depth (SRv) of 0.3μm or less, central plane area factor (SSr) of from 20% to 80%, andaverage wavelength (Sλa) of from 5 μm to 300 μm. For obtaining desiredelectromagnetic characteristics and a friction coefficient, a number ofsurface protrusion of the magnetic layer having sizes (i.e., height) offrom 0.01 μm to 1 μm can be controlled arbitrarily within the range offrom 0 to 2,000 by controlling the surface property due to fillers inthe support, the particle size and the amount of the magnetic powdersadded to the magnetic layer, or by the surface shape of rolls ofcalender treatment. The range of curling is preferably within ±3 mm.

When the magnetic recording medium according to the present inventioncomprises a nonmagnetic layer and a magnetic layer, these physicalproperties in the nonmagnetic layer and the magnetic layer can be variedaccording to purposes. For example, the elastic modulus of the magneticlayer is made higher to improve running durability and at the same timethe elastic modulus of the nonmagnetic layer is made lower than that ofthe magnetic layer to improve the head touching of the magneticrecording medium.

EXAMPLE Examples 1 to 34, Comparative Examples 1 to 5 and ReferenceExamples 1 and 2

Preparation of Coating Solution

Magnetic Coating Solution: ML-1 (acicular magnetic powder was used)

Ferromagnetic metal powder: M-1 100 parts Composition: Co/Fe (atomicratio), 30% Hc: 2,550 Oe Specific surface area: 55 m²/g σ₃: 140 emu/gCrystallite size: 120 Å Long axis length: 0.048 μm Acicular ratio: 4Sintering inhibitor: Al compound (Al/Fe, atomic ratio: 8%) Y compound(Y/Fe, atomic ratio: 6%) Vinyl chloride copolymer 12 parts MR110(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 3 parts UR8200 (manufactured by Toyobo Co., Ltd.) α-Alumina 10 parts HIT55(manufactured by Sumitomo Chemical Co., Ltd.) Average particle size:0.20 μm Specific surface area: 8.0 to 9.0 m²/g Mohs' hardness: 9 pH: 7.7to 9.0 Carbon black 5 parts #50 (manufactured by Asahi Carbon Co., Ltd.)Average particle size: 94 nm Specific surface area: 28 m²/g DBP oilabsorption: 61 ml/100 g pH: 7.5 Volatile content: 1.0 wt %Phenylphosphonic acid 3 parts Butyl stearate 10 parts Butoxyethylstearate 5 parts Isohexadecyl stearate 3 parts Stearic acid 2 partsMethyl ethyl ketone 180 parts Cyclohexanone 180 parts Magnetic CoatingSolution: ML-2 (acicular magnetic powder was used) Ferromagnetic metalpowder: M-2 100 parts Composition: Co/Fe (atomic ratio), 30% Hc: 2,360Oe Specific surface area: 49 m²/g σ₃: 146 emu/g Crystallite size: 170 ÅAverage long axis length: 0.100 μm Acicular ratio: 6 SFD: 0.51 Sinteringinhibitor: Al compound (Al/Fe, atomic ratio: 5%) Y compound (Y/Fe,atomic ratio: 5%) pH: 9.4 Vinyl chloride copolymer 10 parts MR110(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 4 parts UR5500 (manufactured by Toyobo Co., Ltd.) α-Alumina 10 parts HIT70(manufactured by Sumitomo Chemical Co., Ltd.) Average particle size:0.15 μm Specific surface area: 17 m²/g Mohs' hardness: 9 pH: 7.7 to 9.0Carbon black 1 part #50 (manufactured by Asahi Carbon Co., Ltd.) Averageparticle size: 94 nm Specific surface area: 28 m²/g DBP oil absorption:61 ml/100 g pH: 7.5 Volatile content: 1.0 wt % Phenylphosphonic acid 3parts Oleic acid 1 part Stearic acid 0.6 part Ethylene glycol dioleyl 12parts Methyl ethyl ketone 180 parts Cyclohexanone 180 parts MagneticCoating Solution: ML-3 (acicular magnetic powder was used) Ferromagneticmetal powder: M-3 100 parts Composition: Fe/Ni, 96/4 Hc: 1,600 OeSpecific surface area: 45 m²/g Crystallite size: 220 Å σ₃: 135 emu/gAverage long axis length: 0.20 μm Acicular ratio: 9 Vinyl chloridecopolymer 12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)Polyurethane resin 5 parts UR 8600 (manufactured by Toyobo Co., Ltd.)α-Alumina (particle size: 0.65 μm) 2 parts Chromium oxide (particlesize: 0.35 μm) 15 parts Carbon black (particle size: 0.03 μm) 2 partsCarbon black (particle size: 0.3 μm) 9 parts Isohexadecyl stearate 4parts n-Butyl stearate 4 parts Butoxyethyl stearate 4 parts Oleic acid 1part Stearic acid 1 part Methyl ethyl ketone 300 parts Magnetic CoatingSolution: ML-4 (tabular magnetic powder was used) Barium ferritemagnetic powder: M-4 100 parts Composition of molar ratio based on Ba:Fe, 9.10, Co, 0.20, Zn, 0.77 Hc: 2,500 Oe Specific surface area: 50 m²/gσ₃: 58 emu/g Average tabular diameter: 35 nm Tabular ratio: 4 Vinylchloride copolymer 12 parts MR110 (manufactured by Nippon Zeon Co.,Ltd.) Polyurethane resin 3 parts UR 8200 (manufactured by Toyobo Co.,Ltd.) α-Alumina 10 parts HIT55 (manufactured by Sumitomo Chemical Co.,Ltd.) Average particle size: 0.20 μm Specific surface area: 8.0 to 9.0m²/g Mohs' hardness: 9 pH: 7.7 to 9.0 Carbon black 5 parts #50(manufactured by Asahi Carbon Co., Ltd.) Average particle size: 94 nmSpecific surface area: 28 m²/g DBP oil absorption: 61 ml/100 g pH: 7.5Volatile content: 1.0 wt % Phenylphosphonic acid 3 parts Butyl stearate10 parts Butoxyethyl stearate 5 parts Isohexadecyl stearate 3 partsStearic acid 2 parts Methyl ethyl ketone 125 parts Cyclohexanone 125parts Magnetic Coating Solution: ML-5 (tabular magnetic powder was used)Barium ferrite magnetic powder: M-5 100 parts Composition of molar ratiobased on Ba: Fe, 9.10, Co, 0.20, Zn, 0.77 Hc: 2,500 Oe Specific surfacearea: 50 m²/g σ₃: 58 emu/g Average tabular diameter: 35 nm Tabularratio: 2.5 Vinyl chloride copolymer 10 parts MR110 (manufactured byNippon Zeon Co., Ltd.) Polyurethane resin 4 parts UR 5500 (manufacturedby Toyobo Co., Ltd.) α-Alumina 10 parts HIT55 (manufactured by SumitomoChemical Co., Ltd.) Average particle size: 0.20 μm Specific surfacearea: 8.0 to 9.0 m²/g Mohs' hardness: 9 pH: 7.7 to 9.0 Carbon black 1part #50 (manufactured by Asahi Carbon Co., Ltd.) Average particle size:94 nm Specific surface area: 28 m²/g DBP oil absorption: 61 ml/100 g pH:7.5 Volatile content: 1.0 wt % Phenylphosphonic acid 3 parts Oleic acid1 part Stearic acid 0.6 part Ethylene glycol dioleyl 16 parts Methylethyl ketone 180 parts Cyclohexanone 180 parts Magnetic CoatingSolution: ML-6 (acicular magnetic powder was used) Ferromagnetic metalpowder: M-2 100 parts Composition: Co/Fe (atomic ratio), 30% Hc: 2,360Oe Specific surface area: 49 m²/g σ₃: 146 emu/g Crystallite size: 170 ÅAverage long axis length: 0.100 μm Acicular ratio: 6 SFD: 0.51 Sinteringinhibitor: Al compound (Al/Fe, atomic ratio: 5%) Y compound (Y/Fe,atomic ratio: 5%) pH: 9.4 Vinyl chloride copolymer 10 parts MR110(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 4 parts UR5500 (manufactured by Toyobo Co., Ltd.) α-Alumina 10 parts HIT70(manufactured by Sumitomo Chemical Co., Ltd.) Average particle size:0.15 μm Specific surface area: 17 m²/g Mohs' hardness: 9 pH: 7.7 to 9.0Carbon black 1 part #50 (manufactured by Asahi Carbon Co., Ltd.) Averageparticle size: 94 nm Specific surface area: 28 m²/g DBP oil absorption:61 ml/100 g pH: 7.5 Volatile content: 1.0 wt % Phenylphosphonic acid 3parts Myristic acid 1 part Stearic acid 0.6 part Butyl stearate 4 partsCetyl palmitate 4 parts Oleyl oleate 4 parts Methyl ethyl ketone 180parts Cyclohexanone 180 parts Magnetic Coating Solution: ML-7 (acicularmagnetic powder was used) Ferromagnetic metal powder: M-2 100 partsComposition: Co/Fe (atomic ratio), 30% Hc: 2,360 Oe Specific surfacearea: 49 m²/g σ₃: 146 emu/g Crystallite size: 170 Å Average long axislength: 0.100 μm Acicular ratio: 6 SFD: 0.51 Sintering inhibitor: Alcompound (Al/Fe, atomic ratio: 5%) Y compound (Y/Fe, atomic ratio: 5%)pH: 9.4 Vinyl chloride copolymer 10 parts MR110 (manufactured by NipponZeon Co., Ltd.) Polyurethane resin 4 parts UR 5500 (manufactured byToyobo Co., Ltd.) α-Alumina 10 parts HIT70 (manufactured by SumitomoChemical Co., Ltd.) Average particle size: 0.15 μm Specific surfacearea: 17 m²/g Mohs' hardness: 9 pH: 7.7 to 9.0 Carbon black 1 part 50(manufactured by Asahi Carbon Co., Ltd.) Average particle size: 94 nmSpecific surface area: 28 m²/g DBP oil absorption: 61 ml/100 g pH: 7.5Volatile content: 1.0 wt % Phenylphosphonic acid 3 parts Amyl stearate 4parts Butoxyethyl stearate 6 parts Oleyl oleate 4 parts Methyl ethylketone 180 parts Cyclohexanone 180 parts Magnetic Coating Solution: ML-8(acicular magnetic powder was used) Ferromagnetic metal powder: M-2 100parts Composition: Co/Fe (atomic ratio), 30% Hc: 2,360 Oe Specificsurface area: 46 m²/g σ₃: 153 emu/g Crystallite size: 160 Å Average longaxis length: 0.100 μm Acicular ratio: 6 SFD: 0.51 pH: 9.4 Sinteringinhibitor: Al compound (Al/Fe, atomic ratio: 11%) Y compound (Y/Fe,atomic ratio: 7%) Mg compound (Mg/Fe, atomic ratio: 1%) Vinyl chloridecopolymer 10 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)Polyurethane resin 4 parts UR 5500 (manufactured by Toyobo Co., Ltd.)α-Alumina 10 parts HIT55 (dispersion product of 5 parts/ 1 part/4 partsof HIT55/MR110/MEK, which were previously dispersed, manufactured bySumitomo Chemical Co., Ltd.) Average particle size: 0.20 μm Specificsurface area: 8.0 to 9.0 m²/g Mohs' hardness: 9 pH: 7.7 to 9.0 Diamond 1part LS600F (manufactured by LANDS SUPERABRASIVES, Co.) Average particlesize: 0.27 μm Carbon black 1 part 50 (manufactured by Asahi Carbon Co.,Ltd.) Average particle size: 94 nm Specific surface area: 28 m²g DBP oilabsorption: 61 ml/100 g pH: 7.5 Volatile content: 1.0 wt %Phenylphosphonic acid 3 parts Stearic acid 1 part Oleic acid 1 partButyl stearate 4 parts Butoxyethyl stearate 4 parts Neopentyl glycoldioleyl 2 parts Ethylene glycol dioleyl 2 parts Methyl ethyl ketone 180parts Cyclohexanone 180 parts Nonmagnetic Coating Solution: NU-1(spherical inorganic powder was used) Nonmagnetic powder, TiO₂, crystalsystem 80 parts rutile Average-primary particle size: 0.035 μm Specificsurface area (S_(BET)): 40 m²/g pH: 7 TiO₂ content: 90% or more DBP oilabsorption: 27 to 38 ml/100 g Surface-covering compound: Al₂O₃, 8 wt %based on total particles Carbon black 20 parts CONDUCTEX SC-U(manufactured by Columbia Carbon Co., Ltd.) Average primary particlesize: 20 mμ DBP oil absorption: 115 ml/100 g pH: 7.0 Specific surfacearea (S_(BET)): 220 m²/g Volatile content: 1.5% Vinyl chloride copolymer12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethaneresin 5 parts UR 8200 (manufactured by Toyobo co., Ltd.)Phenylphosphonic acid 4 parts Butyl stearate 10 parts Butoxyethylstearate 5 parts Isohexadecyl stearate 2 parts Stearic acid 3 partsMethyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)Nonmagnetic Coating Solution: NU-2 (spherical inorganic powder was used)Nonmagnetic powder, TiO₂, crystal system 100 parts rutile Averageprimary particle size: 0.035 μm Specific surface area (S_(BET)): 40 m²/gpH: 7 TiO₂ content: 90% or more DBP oil absorption: 27 to 38 ml/100 gSurface-covering compound: Al₂O₃ and SiO₂ Ketjen Black EC 13 parts(manufactured by Akzo Nobel Co., Ltd.) Average primary particle size: 30mμ DBP oil absorption: 350 ml/100 g pH: 9.5 Specific surface area(S_(BET)): 950 m²/g Volatile content: 1.0% Vinyl chloride copolymer 16parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 6parts UR 8200 (manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4parts Ethylene glycol dioleyl 16 parts Oleic acid 1 part Stearic acid0.8 part Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)Nonmagnetic Coating Solution: NU-3 (spherical inorganic powder was used)Nonmagnetic powder, TiO₂, crystal system 75 parts rutile Average primaryparticle size: 0.035 μm Specific surface area: 40 m²/g pH: 7 TiO₂content: 90% or more DBP oil absorption: 27 to 38 ml/100 gSurface-covering compound: Al₂O₃ and SiO₂ Carbon black 10 parts KetjenBlack EC (manufactured by Akzo Nobel Co., Ltd.) Average primary particlesize: 30 mμ DBP oil absorption: 350 ml/100 g pH: 9.5 Specific surfacearea (S_(BET)): 950 m²/g Volatile content: 1.0% α-Alumina 15 partsAKP-15 (manufactured by Sumitomo Chemical Co., Ltd.) Average particlesize: 0.65 μm Vinyl chloride copolymer 12 parts MR110 (manufactured byNippon Zeon Co., Ltd.) Polyurethane resin 5 parts UR 8600 (manufacturedby Toyobo Co., Ltd.) Isohexadecyl stearate 4 parts n-Butyl stearate 4parts Butoxyethyl stearate 4 parts Oleic acid 1 part Stearic acid 1 partMethyl ethyl ketone 300 parts Nonmagnetic Coating Solution NU-4(acicular inorganic powder was used) Nonmagnetic powder, α-Fe₂O₃,hematite 80 parts Average long axis length: 0.15 μm Specific surfacearea (S_(BET)): 50 m²/g pH: 9 Surface-covering compound: Al₂O₃, 8 wt %based on total particles Carbon black 20 parts CONDUCTEX SC-U(manufactured by Columbia Carbon Co., Ltd.) Average primary particlesize: 20 mμ DBP oil absorption: 115 ml/100 g pH: 7.0 Specific surfacearea (S_(BET)): 220 m²/g Volatile content: 1.5% Vinyl chloride copolymer12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethaneresin 5 parts UR 8200 (manufactured by Toyobo Co., Ltd.)Phenylphosphonic acid 4 parts Butyl stearate 10 parts Butoxyethylstearate 5 parts Isohexadecyl stearate 2 parts Stearic acid 3 partsMethyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)Nonmagnetic Coating Solution NU-5 (acicular inorganic powder was used)Nonmagnetic powder, α-Fe₂O₃, hematite 100 parts Average long axislength: 0.15 μm Specific surface area (S_(BET)): 50 m²/g pH: 9Surface-covering compound: Al₂O₃, 8 wt % based on total particles Carbonblack 18 parts #3250B (manufactured by Mitsubishi Kasei Corp.) Averageparticle size: 30 nm Specific surface area: 245 m²/g DBP oil absorption:155 ml/100 g pH: 6.0 Volatile content: 1.5 wt % Vinyl chloride copolymer15 parts MR104 (manufactured by Nippon Zeon Co., Ltd.) Polyurethaneresin 7 parts UR 5500 (manufactured by Toyobo Co., Ltd.)Phenylphosphonic acid 4 parts Ethylene glycol dioleyl 16 parts Oleicacid 1.3 parts Stearic acid 0.8 part Methyl ethyl ketone/cyclohexanone250 parts (8/2 mixed solvent) Nonmagnetic Coating Solution NU-6(acicular inorganic powder was used) Nonmagnetic powder, α-Fe₂O₃,hematite 100 parts Average long axis length: 0.15 μm Specific surfacearea (S_(BET)): 50 m²/g pH: 9 Surface-covering compound: Al₂O₃, 8 wt %based on total particles Carbon black 18 parts #3250B (manufactured byMitsubishi Kasei Corp.) Average particle size: 30 nm Specific surfacearea: 245 m²/g DBP oil absorption: 155 ml/100 g pH: 6.0 Volatilecontent: 1.5 wt % Vinyl chloride copolymer 15 parts MR104 (manufacturedby Nippon Zeon Co., Ltd.) Polyurethane resin 7 parts UR 5500(manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4 partsMyristic acid 1 part Stearic acid 0.6 part Butyl stearate 4 parts Cetylpalmitate 4 parts Oleyl oleate 4 parts Methyl ethyl ketone/cyclohexanone250 parts (8/2 mixed solvent) Nonmagnetic Coating Solution NU-7(acicular inorganic powder was used) Nonmagnetic powder, α-Fe₂O₃,hematite 100 parts Average long axis length: 0.15 μm Specific surfacearea (S_(BET)): 50 m²/g pH: 9 Surface-covering compound: Al₂O₃, 8 wt %based on total particles Carbon black 10 parts CONDUCTEX SC-U(manufactured by Columbia Carbon Co., Ltd.) Average primary particlesize: 20 mμ DBP oil absorption: 115 ml/100 g pH: 7.0 Specific surfacearea (S_(BET)): 220 m²/g Volatile content: 1.5% Carbon black 10 parts#50 (manufactured by Asahi Carbon Co., Ltd.) Average particle size: 94nm Specific surface area: 28 m²/g DBP oil absorption: 61 ml/100 g pH:7.5 Volatile content: 1.0 wt % Vinyl chloride copolymer 15 parts MR104(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 7 parts UR5500 (manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4 partsAmyl stearate 4 parts Butoxyethyl stearate 6 parts Oleyl oleate 4 partsMethyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)Nonmagnetic Coating Solution NU-6 (acicular inorganic powder was used)Nonmagnetic powder, α-Fe₂O₃, hematite 100 parts Average long axislength: 0.16 μm Specific surface area (S_(BET)): 50 m²/g pH: 9Surface-covering compound: Al₂O₃, 8 wt % based on total particles Carbonblack 25 parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co.,Ltd.) Average primary particle size: 20 mμ DBP oil absorption: 115ml/100 g pH: 7.0 Specific surface area (S_(BET)): 220 m²/g Volatilecontent: 1.5% Vinyl chloride copolymer 16 parts MR104 (manufactured byNippon Zeon Co., Ltd.) Polyurethane resin 7 parts UR 5500 (manufacturedby Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts Stearic acid 1 partOleic acid 1 part Butyl stearate 4 parts Butoxyethyl stearate 4 partsNeopentyl glycol dioleyl 2 parts Ethylene glycol dioleyl 2 parts Methylethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)

Preparation Method 1 (disc: W/W)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer was blended in a kneader, then dispersedwith a sand mill. Polyisocyanate was added to each resulting dispersionsolution, in an amount of 10 parts to the nonmagnetic layer coatingsolution, and 10 parts to the magnetic layer coating solution. Further,40 parts of cyclohexanone was added to each solution. Each solution wasfiltered through a filter having an average pore diameter of 1 μm toobtain coating solutions for forming the nonmagnetic layer and themagnetic layer.

These coating solutions were respectively simultaneouslymultilayer-coated on a polyethylene terephthalate support having athickness of 62 μm and a central plane average surface roughness of 3 nmof the surface side on which a magnetic layer was to be coated. Thenonmagnetic layer coating solution was coated in a dry thickness of 1.5μm, immediately thereafter the magnetic layer coating solution wascoated on the nonmagnetic layer so as to give the magnetic layer havingthe thickness of 0.15 μm. The coated layers were subjected to randomorientation while the magnetic layer and the nonmagnetic layer werestill wet by passing through an alternating current magnetic fieldgenerator having two magnetic field intensities of frequency of 50 Hz,magnetic field intensity of 250 Gauss and frequency of 50 Hz, magneticfield intensity of 120 Gauss. After drying, the coated layers weresubjected to calendering treatment with calenders of 7 stages at 90° C.at line pressure of 300 kg/cm. The obtained web was punched to a disc of3.7 inches, the disc was subjected to a surface treatment by abrasives,encased in 3.7 inch cartridge having a liner inside (a zip-disccartridge manufactured by Iomega Co., Ltd., U.S.A.), and equipped thecartridge with prescribed mechanism parts to obtain a 3.7 inch floppydisc. A part of samples was subjected to machine direction orientationusing Co magnets with the same pole and counter positions of 4,000 Gbefore random orientation treatment.

In this case, it is preferred to increase the frequency and magneticfield intensity of the alternating current magnetic field generator soas to achieve finally sufficient random orientation, thereby 98% or moreof orientation ratio can be obtained.

When barium ferrite magnetic powder is used, vertical orientation can beperformed besides the above-described orientation. Further, ifnecessary, discs after being punched may be subjected to posttreatments, e.g., a thermal treatment at high temperature (generallyfrom 50 to 90° C.) to accelerate curing of coated layers, or aburnishing treatment with an abrasive tape to scrape surfaceprotrusions.

Preparation Method 2 (computer tape: W/W)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer were blended in a kneader, thendispersed with a sand mill. Polyisocyanate was added to each resultingdispersion solution, in an amount of 2.5 parts to the nonmagnetic layercoating solution, and 3 parts to the magnetic layer coating solution.Further, 40 parts of cyclohexanone was added to each solution. Eachsolution was filtered through a filter having an average pore diameterof 1 μm to obtain coating solutions for forming the nonmagnetic layerand the magnetic layer.

These coating solutions were respectively simultaneouslymultilayer-coated on an aramide support (trade name: Mictron) having athickness of 4.4 μm and a central plane average surface roughness of 2nm of the surface side on which a magnetic layer was to be coated. Thenonmagnetic layer coating solution was coated in a dry thickness of 1.7μm, immediately thereafter the magnetic layer coating solution wascoated on the nonmagnetic layer so as to give the magnetic layer havinga thickness of 0.15 μm. The coated layers were oriented with a cobaltmagnet having a magnetic force of 6,000 G and a solenoid having amagnetic force of 6,000 G while both layers were still wet. Afterdrying, the coated layers were subjected to calendering treatment withcalenders of 7 stages comprising metal rolls at 85° C. at a rate of 200m/min. Subsequently, a backing layer (100 parts of a carbon black havingan average particle size of 17 mμ, 80 parts of calcium carbonate havingan average particle size of 40 mμ, and 5 parts of α-alumina having anaverage particle size of 200 mμ were dispersed in a nitrocelluloseresin, a polyurethane resin and a polyisocyanate) having a thickness of0.5 μm was coated. The obtained web was slit to a width of 3.8 mm. Themagnetic layer surface of the thus-produced tape was cleaned with a tapecleaning apparatus of a nonwoven fabric and a razor blade pressedagainst the surface of the tape, which was attached to a machine havingdelivery and winding-up movement of a slit product. The thus-obtainedmagnetic tape was incorporated in a cartridge for DDS.

Preparation Method 3 (disc: W/D)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer was blended in a kneader, then dispersedwith a sand mill. Polyisocyanate was added to each resulting dispersionsolution, in an amount of 10 parts to the nonmagnetic layer coatingsolution, and 10 parts to the magnetic layer coating solution. Further,40 parts of cyclohexanone was added to each solution. Each solution wasfiltered through a filter having an average pore diameter of 1 μm toobtain coating solutions for forming the nonmagnetic layer and themagnetic layer.

The nonmagnetic layer coating solution was coated in a dry thickness of1.5 μm on a polyethylene terephthalate support having a thickness of 62μm and a central plane average surface roughness of 3 nm of the surfaceside on which the magnetic layer was to be coated, dried, and subjectedto calendering treatment. The magnetic layer coating solution was coatedby blade coating on the nonmagnetic layer so as to give the magneticlayer having the thickness of 0.15 μm. The coated layers were subjectedto random orientation by passing through an alternating current magneticfield generator having two magnetic field intensities of frequency of 50Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz,magnetic field intensity of 120 Gauss. The procedure was carried out inthe same manner as in Preparation Method 1 hereafter. Calendering of thenonmagnetic layer may be omitted.

Preparation Method 4 (computer tape: W/D)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer were blended in a kneader, thendispersed with a sand mill. Polyisocyanate was added to each resultingdispersion solution, in an amount of 2.5 parts to the nonmagnetic layercoating solution, and 3 parts to the magnetic layer coating solution.Further, 40 parts of cyclohexanone was added to each solution. Eachsolution was filtered through a filter having an average pore diameterof 1 μm to obtain coating solutions for forming the nonmagnetic layerand the magnetic layer.

The nonmagnetic layer coating solution was coated in a dry thickness of1.7 μm on an aramide support (trade name: Mictron) having a thickness of4.4 μm and a central plane average surface roughness of 2 nm of thesurface side on which a magnetic layer was to be coated, dried, andsubjected to calendering treatment. The magnetic layer coating solutionwas coated by blade coating on the nonmagnetic layer so as to give themagnetic layer having the thickness of 0.15 μm. The coated layers wereoriented with a cobalt magnet having a magnetic force of 6,000 G and asolenoid having a magnetic force of 6,000 G. The procedure was carriedout in the same manner as in Preparation Method 2 hereafter. Calenderingof the nonmagnetic layer may be omitted.

Preparation Method 5 (disc: spin coating)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer were blended in a kneader, thendispersed with a sand mill. Polyisocyanate was added to each resultingdispersion solution, in an amount of 10 parts to the nonmagnetic layercoating solution, and 10 parts to the magnetic layer coating solution.Further, 40 parts of cyclohexanone was added to each solution. Eachsolution was filtered through a filter having an average pore diameterof 1 μm to obtain coating solutions for forming the nonmagnetic layerand the magnetic layer.

The nonmagnetic layer coating solution was coated in a dry thickness of1.5 μm by spin coating on a polyethylene terephthalate support having athickness of 62 μm and a central plane average surface roughness of 3 nmof the surface side on which a magnetic layer was to be coated anddried. The magnetic layer coating solution was coated by spin coating onthe nonmagnetic layer so as to give the magnetic layer having thethickness of 0.15 μm. The coated layers were oriented using a Co magnetswith the same pole and counter positions of 6,000 G in thecircumferential direction and the surface of the layer was smoothed bybatch system rolling treatment by which the same pressure as inPreparation Method 1 can be applied. The procedure was carried out inthe same manner as in Preparation Method 1 hereafter. Also, the magneticlayer may be coated by spin coating on the nonmagnetic layer while thenonmagnetic layer coated by spin coating is still wet. By using the spincoating process, not only the amount of residual magnetization in therecording direction can be made large but also vertical magnetizationcomponents of barium ferrite and ferromagnetic metal powders of shortacicular ratio can be reduced and symmetric property of reproduced waveform can be improved.

Support B-1: Polyethylene Terephthalate

Thickness: 62 μm

F-5 value:

MD: 114 MPa, TD: 107 MPa

Breaking strength:

MD: 276 MPa, TD: 281 MPa

Breaking extension:

MD: 174%, TD: 139%

Thermal shrinkage factor (80° C., 30 minutes):

MD: 0.04%, TD: 0.05%

Thermal shrinkage factor (100° C., 30 minutes):

MD: 0.2%, TD: 0.3%

Temperature expansion coefficient:

Long axis: 15×10⁻⁶/° C.

Short axis: 18×10⁻⁶/° C.

Central plane average surface roughness: 3 nm

Thickness unevenness: 1.5%

Glass transition temperature (Tg): 69° C.

Melting point: 263° C.

Support B-2: Polyethylene Terephthalate

Thickness: 62 μm

Thickness unevenness: 4.0%

Support B-3: Polyethylene Terephthalate (for comparison)

Thickness: 62 μm

Thickness unevenness: 7.2%

Support B-4: Polyethylene Naphthalate

Thickness: 55 μm

F-5 value:

MD: 155 MPa, TD: 155 MPa

Breaking strength:

MD: 310 MPa, TD: 320 MPa

Breaking extension:

MD: 90%, TD: 92%

Thermal shrinkage factor (80° C., 30 minutes):

MD: 0.00%, TD: 0.00%

Thermal shrinkage factor (100° C., 30 minutes):

MD: 0.1%, TD: 0.00%

Temperature expansion coefficient:

Long axis: 10×10⁻⁶/° C.

Short axis: 11×10⁻⁶/° C.

Central plane average surface roughness: 1.8 nm

Thickness unevenness: 4.8%

Glass transition temperature (Tg): 113° C.

Melting point: 273° C.

Support B-5: Aramide

Thickness: 4.4 μm

F-5 value:

MD: 320 MPa, TD: 420 MPa

Breaking strength:

MD: 460 MPa, TD: 280 MPa

Breaking extension:

MD: 50%, TD: 30%

Thermal shrinkage factor (150° C., 30 minutes):

MD: 0.3%, TD: 0.1%

Central plane average surface roughness: 2 nm

Thickness unevenness: 4.8%

Support B-6: Aramide (for comparison)

Thickness: 4.4 μm

Central plane average surface roughness: 2 nm

Thickness unevenness: 8.0%

Orientation

O-1: Random orientation

O-2: Orientation in the machine direction using a Co magnet first, thenrandom orientation

O-3: Orientation in the machine direction using a Co magnet first, thenin the machine direction using a solenoid

O-4: Orientation in the vertical direction using a Co magnet

O-5: Orientation in the circumferential direction using a Co magnet

Backing Layer Coating Solution: BL-1 Fine carbon black powder 100 partsBP-800 (average particle size: 17 mμ, manufactured by Cabot Co., Ltd.)Coarse carbon black powder 10 parts Thermal Black (average particlesize: 270 mμ, manufactured by Cancarb Co., Ltd.) Calcium carbonate (softinorganic powder) 80 parts Hakuenka O (average particle size: 40 mμ,Mohs' hardness: 3, manufactured by Shiraishi Kogyo Co., Ltd.) α-Alumina(hard inorganic powder) 5 parts (average particle size: 200 mμ, Mohs'hardness: 9) Nitrocellulose resin 140 parts Polyurethane resin 15 partsPolyisocyanate 40 parts Polyester resin 5 parts Dispersant: Copperoleate 5 parts Copper phthalocyanine 5 parts Barium sulfate 5 partsMethyl ethyl ketone 2,200 parts Butyl acetate 300 parts Toluene 600parts

The above compositions of the coating solution for the backing layerwere blended in a continuous kneader, then dispersed with a sand mill.The resulting dispersion solution was filtered through a filter havingan average pore diameter of 1 μm to obtain a coating solution forforming the backing layer.

With respect to samples obtained by combining the above-described eachmethod arbitrarily as shown in Table 1 or 3, magnetic characteristics,central plane average surface roughness, areal recording density, etc.,were determined and the results obtained are shown in Table 2 or 4.

(1) Magnetic Characteristics (Hc):

Magnetic characteristics were measured using a vibrating samplemagnetometer (a product of Toei Kogyo Co., Ltd.) at Hm 10 KOe.

(2) Central Plane Average Surface Roughness (Ra):

Surface roughness (Ra) of the area of about 250 μm×250 μm, Rrms,peak-valley values were measured using “TOPO3D” (a product of WYKO Co.,Ltd., U.S.A.) by 3D-MIRAU method. The wavelength of measurement wasabout 650 nm and spherical compensation and cylindrical compensationwere applied. Measurement was performed using a light interference typenon-contact surface roughness meter.

(3) Areal Recording Density:

Areal recording density means a value obtained by multiplying linearrecording density by track density.

(4) Linear Recording Density:

Linear recording density means a bit number of signals recorded per 1inch in the recording direction.

(5) Track Density:

Track density means a track number per 1 inch.

(6) φm:

φm is the amount of magnetization per unit area of a magnetic recordingmedium, which is represented by Bm (Gauss) multiplying thickness. Thisis the value obtained by using a vibrating sample magnetometer (aproduct of Toei Kogyo Co., Ltd.) at Hm 10 KOe, which can be directlymeasured.

(7) Error Rate of Tape:

The above signals of linear recording density were recorded on the tapeby 8-10 conversion PR1 equalization system and error rate of the tapewas measured using a DDS drive.

(8) Error Rate of Disc:

The above signals of linear recording density were recorded on the discby (2,7) RLL modulation system and error rate of the disc was measured.

(9) Thickness of Magnetic Layer:

The sample having the thickness of about 0.1 μm was cut out with adiamond cutter in the machine direction of the magnetic medium, observedwith a transmission type electron microscope of from 10,000 to 100,000,preferably from 20,000 to 50,000 magnifications and photographed. Theprint size of the photograph was from A4 (i.e., 297×210 mm) to A5 (i.e.,210×148 mm) sizes. The present inventors paid attentions to thedifference of the shapes of the ferromagnetic powders and thenonmagnetic powders of the magnetic layer and the nonmagnetic layer andrimmed the interface of the magnetic layer and the nonmagnetic layer andalso the surface of the magnetic layer with black color by visualjudgement. Thereafter, the distance of the rimmed lines was measured bythe image processing apparatus “IBAS2” (manufactured by Zeiss Corp.).Measurement was conducted from 85 to 300 times when the length of thesample photograph was 21 cm. The average measured value at that time wastaken as σ, and the standard deviation of the measured value was takenas σ. d depended on the description in JP-A-5-298653 and σ was obtainedby equation (2) in JP-A-5-298653. di means each measured value and n isfrom 85 to 300.

(10) Thickness of Support:

The thickness of the support was measured according to theabove-described method.

(11) Thickness Unevenness of Support:

The thickness unevenness of the support Δt was measured according to theabove-described method.

(12) Measurement of C/Fe Ratio:

C/Fe value was determined using Auger electron spectrometer PHI-660 typemanufactured by Φ Co. Conditions of measurement were as follows.

Accelerating voltage of primary electron beam: 3 KV

Electric current of the sample: 130 nA

Magnification: 250-fold

Inclination angle: 30°

The C/Fe ratio is obtained as the C/Fe peak by integrating the valuesobtained by the above conditions in the region of kinetic energy of 130eV to 730 eV three times and finding the strengths of KLL peak of thecarbon and LMM peak of the iron as differentials.

TABLE 1 Disc Magnetic Layer Thick- Surface Prepara- Prescrip- ness HcRoughness φm Lower tion Orien- Sample No. tion (μm) (Oe) (nm) (emu/cm²)Layer Support Method tation Example 1 ML-2 0.15 2,360 3.5 4.8 × 10⁻³NU-1 B-1 Method 1 O-1 Example 2 ML-2 0.15 2,360 2.3 4.8 × 10⁻³ NU-2 B-1Method 1 O-1 Example 3 ML-2 0.15 2,360 1.9 4.8 × 10⁻³ NU-4 B-1 Method 1O-1 Example 4 ML-2 0.15 2,360 1.7 4.8 × 10⁻³ NU-5 B-1 Method 1 O-1Example 5 ML-2 0.05 2,400 1.4 1.6 × 10⁻³ NU-5 B-1 Method 1 O-1 Example 6ML-2 0.10 2,380 1.6 3.2 × 10⁻³ NU-5 B-1 Method 1 O-1 Example 7 ML-2 0.202,330 1.9 6.4 × 10⁻³ NU-5 B-1 Method 1 O-1 Example 8 ML-2 0.15 2,360 3.54.8 × 10⁻³ NU-1 B-2 Method 1 O-1 Example 9 ML-1 0.15 2,550 2.5 4.2 ×10⁻³ NU-5 B-1 Method 1 O-1 Comparative ML-3 0.15 1,600 3.1 4.8 × 10⁻³NU-5 B-1 Method 1 O-1 Example 1 Example 10 ML-4 0.15 2,500 2.2 2.1 ×10⁻³ NU-5 B-1 Method 1 O-1 Example 11 ML-5 0.15 2,500 1.8 2.4 × 10⁻³NU-5 B-1 Method 1 O-1 Example 12 ML-2 0.15 2,360 2.5 4.8 × 10⁻³ NU-5 B-1Method 3 O-1 Example 13 ML-2 0.15 2,360 1.7 4.8 × 10⁻³ NU-5 B-1 Method 1O-2 Example 14 ML-5 0.15 2,500 1.8 2.5 × 10⁻³ NU-5 B-1 Method 1 O-2Example 15 ML-4 0.15 2,700 1.9 2.3 × 10⁻³ NU-5 B-1 Method 1 O-4 Example16 ML-2 0.15 2,660 1.6 4.8 × 10⁻³ NU-5 B-1 Method 5 O-5 Example 17 ML-40.15 2,700 1.8 2.3 × 10⁻³ NU-5 B-1 Method 5 O-5 Example 21 ML-6 0.152,360 1.7 4.8 × 10⁻³ NU-6 B-1 Method 1 O-1 Example 22 ML-7 0.15 2,3601.7 4.8 × 10⁻³ NU-7 B-1 Method 1 O-1 Example 23 ML-8 0.15 2,360 1.7 4.8× 10⁻³ NU-8 B-4 Method 1 O-1 Comparative ML-2 0.15 2,360 3.5 4.8 × 10⁻³NU-1 B-3 Method 1 O-1 Example 2

TABLE 2 Linear Areal Track Recording Recording Error Density DensityDensity Rate Sample No. (tpi) (kbpi) (G bit/inch²) (10⁻⁵) C/Fe Example 15,200 144 0.75 0.2 40 Example 2 5,200 144 0.75 0.08 10 Example 3 5,200144 0.75 0.03 70 Example 4 5,200 144 0.75 0.01 25 Example 5 5,200 1440.75 0.06 25 Example 6 5,200 144 0.75 0.01 25 Example 7 5,200 144 0.750.2 25 Example 8 5,200 144 0.75 0.008 25 Example 9 5,200 144 0.75 0.00430 Comparative 5,200 144 0.75 40 — Example 1 Example 10 5,200 144 0.750.01 — Example 11 5,200 144 0.75 0.005 — Example 12 5,200 144 0.75 0.125 Example 13 5,200 144 0.75 0.001 25 Example 14 5,200 144 0.75 0.0006 —Example 15 5,200 144 0.75 0.0004 — Example 16 5,200 144 0.75 0.0002 25Example 17 5,200 144 0.75 0.0001 — Example 18 7,500 200 1.5 0.8 —Example 19 6,000 166 1.0 0.08 — Example 20 3,000 120 0.36 0.007 —Example 21 5,200 144 0.75 0.01 45 Example 22 5,200 144 0.75 0.01 60Example 23 5,200 144 0.75 0.01 — Comparative 5,200 144 0.75 1.5 40Example 2 Reference 2,000  50 0.1 0.5 — Example 1

In each of Examples 18 to 20 and Reference Example 1, the disc inExample 13 was used and error rate was determined with varying linearrecording density and track density.

TABLE 3 Computer Tape Magnetic Layer Thick- Surface Prepara- Prescrip-ness Hc Roughness φm Lower tion Orien- Sample No. tion (μm) (Oe) (nm)(emu/cm²) Layer Support Method tation Example 24 ML-2 0.15 2,460 3.7 4.8× 10⁻³ NU-1 B-5 Method 2 O-3 Example 25 ML-2 0.15 2,460 2.4 4.8 × 10⁻³NU-2 B-5 Method 2 O-3 Example 26 ML-2 0.15 2,460 2.1 4.8 × 10⁻³ NU-4 B-5Method 2 O-3 Example 27 ML-2 0.15 2,460 1.8 4.8 × 10⁻³ NU-5 B-5 Method 2O-3 Example 28 ML-2 0.05 2,500 1.7 1.6 × 10⁻³ NU-5 B-5 Method 2 O-3Example 29 ML-2 0.10 2,480 1.7 3.2 × 10⁻³ NU-5 B-5 Method 2 O-3 Example30 ML-2 0.20 2,430 2.0 6.4 × 10⁻³ NU-5 B-5 Method 2 O-3 Example 31 ML-10.15 2,650 2.6 4.2 × 10⁻³ NU-5 B-5 Method 2 O-3 Comparative ML-3 0.151,700 3.3 4.8 × 10⁻³ NU-5 B-5 Method 2 O-3 Example 3 Example 32 ML-20.15 2,460 2.7 4.8 × 10⁻³ NU-5 B-5 Method 4 O-3 Comparative ML-2 0.152,460 2.6 4.8 × 10⁻³ NU-2 B-6 Method 2 O-3 Example 4 Comparative ML-20.20 2,430 2.2 6.4 × 10⁻³ NU-4 B-6 Method 2 O-3 Example 5

TABLE 4 Linear Areal Track Recording Recording Error Density DensityDensity Rate Sample No. (tpi) (kbpi) (G bit/inch²) (10⁻⁵) C/Fe Example24 3,000 122 0.366 0.09 40 Example 25 3,000 122 0.366 0.02 10 Example 263,000 122 0.366 0.003 70 Example 27 3,000 122 0.366 0.001 25 Example 283,000 122 0.366 0.01 25 Example 29 3,000 122 0.366 0.002 25 Example 303,000 122 0.366 0.01 25 Example 31 3,000 122 0.366 0.0005 30 Comparative3,000 122 0.366 11 — Example 3 Example 32 3,000 122 0.366 0.02 25Example 33 4,000 150 0.6 0.02 — Example 34 5,000 170 0.85 0.5 —Comparative 3,000 122 0.366 1.5 10 Example 4 Comparative 3,000 122 0.3662 25 Example 5 Reference 3,000  50 0.15 0.1 — Example 2

As described above, the above signals of linear recording density wererecorded on the tape by 8-10 conversion PR1 equalization system anderror rate of the tape was measured using a DDS drive. In each ofExamples 33, 34 and Reference Example 2, the tape in Example 24 was usedand error rate was determined with varying linear recording density andtrack density.

From the results in Tables 2 and 4, it can be seen that the error ratesof the magnetic recording media (i.e., disc and computer tape) accordingto the present invention, in particular, in high density recordingregion, are 1×10⁻⁵ or less, which are conspicuously excellent ascompared with conventional disc-like media.

Example 35

Preparation of Coating Solution Magnetic Coating Solution: mL-2(acicular magnetic powder was used) Ferromagnetic metal powder: m-1 100parts Composition: Co/Fe (atomic ratio), 30% Hc: 2,550 Oe Specificsurface area: 55 m²/g σ₃: 140 emu/g Crystallite size: 120 Å Long axislength: 0.048 μm Acicular ratio: 4 Sintering inhibitor: Al compound(Al/Fe, atomic ratio: 8%) Y compound (Y/Fe, atomic ratio: 6%) Vinylchloride copolymer 12 parts MR110 (manufactured by Nippon Zeon Co.,Ltd.) Polyurethane resin 3 parts UR 8200 (manufactured by Toyobo Co.,Ltd.) α-Alumina 10 parts HIT55 (manufactured by Sumitomo Chemical Co.,Ltd.) Carbon black 5 parts #50 (manufactured by Asahi Carbon Co., Ltd.)Phenylphosphonic acid 3 parts Lubricant Butyl stearate 10 partsButoxyethyl stearate 5 parts Isohexadecyl stearate 3 parts Or estershown in Table 9-1 or 10-1 Stearic acid 2 parts Methyl ethyl ketone 180parts Cyclohexanone 180 parts Magnetic Coating Solution: mL-2 (acicularmagnetic powder was used) Ferromagnetic metal powder: m-2 100 partsComposition: Co/Fe (atomic ratio), 30% Hc: 2,360 Oe Specific surfacearea: 49 m²/g σ₃: 146 emu/g Crystallite size: 170 Å Long axis length:0.100 μm Acicular ratio: 6 SFD: 0.51 Sintering inhibitor: Al compound(Al/Fe, atomic ratio: 5%) Y compound (Y/Fe, atomic ratio: 5%) pH: 9.4Vinyl chloride copolymer 10 parts MR110 (manufactured by Nippon ZeonCo., Ltd.) Polyurethane resin 4 parts UR 5500 (manufactured by ToyoboCo., Ltd.) α-Alumina 10 parts HIT70 (manufactured by Sumitomo ChemicalCo., Ltd.) Carbon black 1 part #50 (manufactured by Asahi Carbon Co.,Ltd.) Phenylphosphonic acid 3 parts Lubricant Ethylene glycol dioleyl 12parts Or ester shown in Table 9-1 or 10-1 Oleic acid 1 part Stearic acid0.6 part Methyl ethyl ketone 180 parts Cyclohexanone 180 parts MagneticCoating Solution: mL-3 (acicular magnetic powder was used) Ferromagneticmetal powder: m-3 100 parts Composition: Fe/Ni, 96/4 Hc: 1,600 OeSpecific surface area: 45 m²/g Crystallite size: 220 Å σ₃: 135 emu/gAverage long axis length: 0.20 μm Acicular ratio: 9 Vinyl chloridecopolymer 12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)Polyurethane resin 5 parts UR 8600 (manufactured by Toyobo Co., Ltd.)α-Alumina (particle size: 0.65 μm) 2 parts Chromium oxide (particlesize: 0.35 μm) 15 parts Carbon black (particle size: 0.03 μm) 2 partsCarbon black (particle size: 0.3 μm) 9 parts Lubricant Butyl stearate 4parts Butoxyethyl stearate 4 parts Isohexadecyl stearate 4 parts Orester shown in Table 9-1 or 10-1 Oleic acid 1 part Stearic acid 1 partMethyl ethyl ketone 300 parts Magnetic Coating Solution: mL-4 (tabularmagnetic powder was used) Barium ferrite magnetic powder: m-4 100 partsComposition of molar ratio based on Ba: Fe, 9.10, Co, 0.20, Zn, 0.77 Hc:2,500 Oe Specific surface area: 50 m²/g σ₃: 58 emu/g Tabular diameter:35 nm Tabular ratio: 4 Vinyl chloride copolymer 12 parts MR110(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 3 parts UR8200 (manufactured by Toyobo Co., Ltd.) α-Alumina 10 parts HIT55(manufactured by Sumitomo Chemical Co., Ltd.) Carbon black 5 parts #50(manufactured by Asahi Carbon Co., Ltd.) Phenylphosphonic acid 3 partsLubricant Butyl stearate 10 parts Butoxyethyl stearate 5 partsIsohexadecyl stearate 3 parts Or ester shown in Table 9-1 or 10-1Stearic acid 2 parts Methyl ethyl ketone 125 parts Cyclohexanone 125parts Magnetic Coating Solution: mL-5 (tabular magnetic powder was used)Barium ferrite magnetic powder: m-5 100 parts Composition of molar ratiobased on Ba: Fe, 9.10, Co, 0.20, Zn, 0.77 Hc: 2,500 Oe Specific surfacearea: 50 m²/g σ₃: 58 emu/g Tabular diameter: 35 nm Tabular ratio: 2.5Vinyl chloride copolymer 10 parts MR110 (manufactured by Nippon ZeonCo., Ltd.) Polyurethane resin 4 parts UR 5500 (manufactured by ToyoboCo., Ltd.) α-Alumina 10 parts HIT55 (manufactured by Sumitomo ChemicalCo., Ltd.) Carbon black 1 part #50 (manufactured by Asahi carbon co.,Ltd.) Phenylphosphonic acid 3 parts Lubricant Ethylene glycol dioleyl 16parts Or ester shown in Table 9-1 or 10-1 Oleic acid 1 part Stearic acid0.6 part Methyl ethyl ketone 180 parts Cyclohexanone 180 partsNonmagnetic Coating Solution: nU-1 (spherical inorganic powder was used)Nonmagnetic powder, TiO₂, crystal system 80 parts rutile Average primaryparticle size: 0.035 μm Specific surface area (S_(BET)): 40 m²/g pH: 7TiO₂ content: 90% or more DBP oil absorption: 27 to 38 ml/100 gSurface-covering compound: Al₂O₃, 8 wt % based on total particles Carbonblack 20 parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co.,Ltd.) Vinyl chloride copolymer 12 parts MR110 (manufactured by NipponZeon Co., Ltd.) Polyurethane resin 5 parts UR 8200 (manufactured byToyobo Co., Ltd.) Phenylphosphonic acid 4 parts Lubricant Butyl stearate10 parts Butoxyethyl stearate 5 parts Isohexadecyl stearate 2 parts Orester shown in Table 9-1 or 10-1 Stearic acid 3 parts Methyl ethylketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic CoatingSolution: nU-2 (spherical inorganic powder was used) Nonmagnetic powder,TiO₂, crystal system 100 parts rutile Average primary particle size:0.035 μm Specific surface area (S_(BET)): 40 m²/g pH: 7 TiO₂ content:90% or more DBP oil absorption: 27 to 38 ml/100 g Surface-coveringcompound: Al₂O₃, and SiO₂ were present on the surfaces of particlesKetjen Black EC 13 parts (manufactured by Akzo Nobel Co., Ltd.) Averageprimary particle size: 30 mμ DBP oil absorption: 350 ml/100 g pH: 9.5Specific surface area (S_(BET)): 950 m²/g Volatile content: 1.0% Vinylchloride copolymer 16 parts MR110 (manufactured by Nippon Zeon Co.,Ltd.) Polyurethane resin 6 parts UR 8200 (manufactured by Toyobo Co.,Ltd.) Phenylphosphonic acid 4 parts Lubricant Ethylene glycol dioleyl 16parts Or ester shown in Table 9-1 or 10-1 Oleic acid 1 part Stearic acid0.8 part Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)Nonmagnetic Coating Solution: nU-3 (spherical inorganic powder was used,comparative example) Nonmagnetic powder, TiO₂, crystal system 75 partsrutile Average primary particle size: 0.035 μm Specific surface area(S_(BET)): 40 m²/g pH: 7 TiO₂ content: 90% or more DBP oil absorption:27 to 38 ml/100 g Surface-covering compound: Al₂O₃, and SiO₂ werepresent on the surfaces of particles Carbon black 10 parts Ketjen BlackEC (manufactured by Akzo Nobel Co., Ltd.) α-Alumina 15 parts AKP-15(manufactured by Sumitomo Chemical Co., Ltd.) Average particle size:0.65 μm Vinyl chloride copolymer 12 parts MR110 (manufactured by NipponZeon Co., Ltd.) Polyurethane resin 5 parts UR 8600 (manufactured byToyobo Co., Ltd.) Lubricant Ester shown in Table 9-1 or 10-1 Oleic acid1 part Stearic acid 1 part Methyl ethyl ketone 300 parts NonmagneticCoating Solution nU-4 (acicular inorganic powder was used) Nonmagneticpowder, α-Fe₂O₃, hematite 80 parts Long axis length: 0.15 μm Specificsurface area (S_(BET)): 50 m²/g pH: 9 Surface-covering compound: Al₂O₃,8 wt % based on total particles Carbon black 20 parts CONDUCTEX SC-U(manufactured by Columbia Carbon Co., Ltd.) Vinyl chloride copolymer 12parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 5parts UR 8200 (manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4parts Lubricant Butyl stearate 10 parts Butoxyethyl stearate 5 partsIsohexadecyl stearate 2 parts Or ester shown in Table 9-1 or 10-1Stearic acid 3 parts Methyl ethyl ketone/cyclohexanone 250 parts (8/2mixed solvent) Nonmagnetic Coating Solution nU-5 (acicular inorganicpowder was used) Nonmagnetic powder, α-Fe₂O₃, hematite 100 parts Longaxis length: 0.15 μm Specific surface area (S_(BET)): 50 m²/g pH: 9Surface-covering compound: Al₂O₃, 8 wt % based on total particles Carbonblack 18 parts #3250B (manufactured by Mitsubishi Kasei Corp.) Vinylchloride copolymer 15 parts MR104 (manufactured by Nippon Zeon Co.,Ltd.) Polyurethane resin 7 parts UR 5500 (manufactured by Toyobo Co.,Ltd.) Phenylphosphonic acid 4 parts Lubricant Ethylene glycol dioleyl 16parts Or ester shown in Table 9-1 or 10-1 Oleic acid 1.3 parts Stearicacid 0.8 part Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixedsolvent)

Preparation Method 1 (disc: W/W)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer was blended in a kneader, then dispersedwith a sand mill. Polyisocyanate was added to each resulting dispersionsolution, in an amount of 10 parts to the nonmagnetic layer coatingsolution, and 10 parts to the magnetic layer coating solution. Further,40 parts of cyclohexanone was added to each solution. Each solution wasfiltered through a filter having an average pore diameter of 1 μm toobtain coating solutions for forming the nonmagnetic layer and themagnetic layer.

These coating solutions were respectively simultaneouslymultilayer-coated on a polyethylene terephthalate support having athickness of 62 μm and a central plane average surface roughness of 3 nmof the surface side on which a magnetic layer was to be coated. Thenonmagnetic layer coating solution was coated in a dry thickness of 1.5μm, immediately thereafter the magnetic layer coating solution wascoated on the nonmagnetic layer so as to give the magnetic layer havingthe thickness of 0.15 μm. The coated layers were subjected to randomorientation while the magnetic layer and the nonmagnetic layer werestill wet by passing through an alternating current magnetic fieldgenerator having two magnetic field intensities of frequency of 50 Hz,magnetic field intensity of 250 Gauss and frequency of 50 Hz, magneticfield intensity of 120 Gauss. After drying, the coated layers weresubjected to calendering treatment with calenders of 7 stages at 90° C.at line pressure of 300 kg/cm. The obtained web was punched to a disc of3.7 inches, the disc was subjected to a surface treatment by abrasives,encased in 3.7 inch cartridge having a liner inside (a zip-disccartridge manufactured by Iomega Co., Ltd., U.S.A.), and equipped thecartridge with prescribed mechanism parts to obtain a 3.7 inch floppydisc. A part of samples was subjected to machine direction orientationusing Co magnets with the same pole and counter positions of 4,000 Gbefore random orientation treatment.

In this case, it is preferred to increase the frequency and magneticfield intensity of the alternating current magnetic field generator soas to achieve finally sufficient random orientation, thereby 98% or moreof orientation ratio can be obtained.

When barium ferrite magnetic powder is used, vertical orientation can beperformed besides the above-described orientation. Further, ifnecessary, discs after being punched may be subjected to posttreatments, e.g., a thermal treatment at high temperature (generallyfrom 50 to 90° C.) to accelerate curing of coated layers, or aburnishing treatment with an abrasive tape to scrape surfaceprotrusions.

Preparation Method 2 (computer tape: W/W)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer were blended in a kneader, thendispersed with a sand mill. Polyisocyanate was added to each resultingdispersion solution, in an amount of 2.5 parts to the nonmagnetic layercoating solution, and 3 parts to the magnetic layer coating solution.Further, 40 parts of cyclohexanone was added to each solution. Eachsolution was filtered through a filter having an average pore diameterof 1 μm to obtain coating solutions for forming the nonmagnetic layerand the magnetic layer.

These coating solutions were respectively simultaneouslymultilayer-coated on an aramide support (trade name: Mictron) having athickness of 4.4 μm and a central plane average surface roughness of 2nm of the surface side on which a magnetic layer was to be coated. Thenonmagnetic layer coating solution was coated in a dry thickness of 1.7μm, immediately thereafter the magnetic layer coating solution wascoated on the nonmagnetic layer so as to give the magnetic layer havinga thickness of 0.15 μm. The coated layers were oriented with a cobaltmagnet having a magnetic force of 6,000 G and a solenoid having amagnetic force of 6,000 G while both layers were still wet. Afterdrying, the coated layers were subjected to calendering treatment withcalenders of 7 stages comprising metal rolls at 85° C. at a rate of 200m/min. Subsequently, a backing layer (100 parts of a carbon black havingan average particle size of 17 mμ, 80 parts of calcium carbonate havingan average particle size of 40 mμ, and 5 parts of α-alumina having anaverage particle size of 200 mμ were dispersed in a nitrocelluloseresin, a polyurethane resin and a polyisocyanate) having a thickness of0.5 μm was coated. The obtained web was slit to a width of 3.8 mm. Themagnetic layer surface of the thus-produced tape was cleaned with a tapecleaning apparatus of a nonwoven fabric and a razor blade pressedagainst the surface of the tape, which was attached to a machine havingdelivery and winding-up movement of a slit product. The thus-obtainedmagnetic tape was incorporated in a cartridge for DDS.

Preparation Method 3 (disc: W/D)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer was blended in a kneader, then dispersedwith a sand mill. Polyisocyanate was added to each resulting dispersionsolution, in an amount of 10 parts to the nonmagnetic layer coatingsolution, and 10 parts to the magnetic layer coating solution. Further,40 parts of cyclohexanone was added to each solution. Each solution wasfiltered through a filter having an average pore diameter of 1 μm toobtain coating solutions for forming the nonmagnetic layer and themagnetic layer.

The nonmagnetic layer coating solution was coated in a dry thickness of1.5 μm on a polyethylene terephthalate support having a thickness of 62μm and a central plane average surface roughness of 3 nm of the surfaceside on which the magnetic layer was to be coated, dried, and subjectedto calendering treatment. The magnetic layer coating solution was coatedby blade coating on the nonmagnetic layer so as to give the magneticlayer having the thickness of 0.15 μm. The coated layers were subjectedto random orientation by passing through an alternating current magneticfield generator having two magnetic field intensities of frequency of 50Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz,magnetic field intensity of 120 Gauss. The procedure was carried out inthe same manner as in Preparation Method 1 hereafter. Calendering of thenonmagnetic layer may be omitted.

Preparation Method 4 (computer tape: W/D)

Each of the above compositions of the coating solutions for the magneticlayer and the nonmagnetic layer were blended in a kneader, thendispersed with a sand mill. Polyisocyanate was added to each resultingdispersion solution, in an amount of 2.5 parts to the nonmagnetic layercoating solution, and 3 parts to the magnetic layer coating solution.Further, 40 parts of cyclohexanone was added to each solution. Eachsolution was filtered through a filter having an average pore diameterof 1 μm to obtain coating solutions for forming the nonmagnetic layerand the magnetic layer.

The nonmagnetic layer coating solution was coated in a dry thickness of1.7 μm on an aramide support (trade name: Mictron) having a thickness of4.4 μm and a central plane average surface roughness of 2 nm of thesurface side on which a magnetic layer was to be coated, dried, andsubjected to calendering treatment. The magnetic layer coating solutionwas coated by blade coating on the nonmagnetic layer so as to give themagnetic layer having the thickness of 0.15 μm. The coated layers wereoriented with a cobalt magnet having a magnetic force of 6,000 G and asolenoid having a magnetic force of 6,000 G. The procedure was carriedout in the same manner as in Preparation Method 2 hereafter. Calenderingof the nonmagnetic layer may be omitted.

Preparation Method 5 (disc: spin coating)

Each of the above ten compositions of the coating solutions for themagnetic layer and the nonmagnetic layer were blended in a kneader, thendispersed with a sand mill. Polyisocyanate was added to each resultingdispersion solution, in an amount of 10 parts to the nonmagnetic layercoating solution, and 10 parts to the magnetic layer coating solution.Further, 40 parts of cyclohexanone was added to each solution. Eachsolution was filtered through a filter having an average pore diameterof 1 μm to obtain coating solutions for forming the nonmagnetic layerand the magnetic layer.

The nonmagnetic layer coating solution was coated in a dry thickness of1.5 μm by spin coating on a polyethylene terephthalate support having athickness of 62 μm and a central plane average surface roughness of 3 nmof the surface side on which a magnetic layer was to be coated anddried. The magnetic layer coating solution was coated by spin coating onthe nonmagnetic layer so as to give the magnetic layer having thethickness of 0.15 μm. The coated layers were oriented using a Co magnetswith the same pole and counter positions of 6,000 G in thecircumferential direction and the surface of the layer was smoothed bybatch system rolling treatment by which the same pressure as inPreparation Method 1 can be applied. The procedure was carried out inthe same manner as in Preparation Method 1 hereafter. Also, the magneticlayer may be coated by spin coating on the nonmagnetic layer while thenonmagnetic layer coated by spin coating is still wet. By using the spincoating process, not only the amount of residual magnetization in therecording direction can be made large but also vertical magnetizationcomponents of barium ferrite and ferromagnetic metal powders of shortacicular ratio can be reduced and symmetric property of reproduced waveform can be improved.

Lubricant: Diester

L-a1: C₁₇H₃₅COO(CH₂)₄OCOC₁₇H₃₅

L-a2: C₁₁H₂₁COO(CH₂)₄OCOC₁₁H₂₁

L-a3: C₁₇H₃₃COO(CH₂)₂OCOC₁₇H₃₃

L-a4: C₁₁H₂₃COO(CH₂)₄OCOC₁₁H₂₃

L-a5: C₂₇H₅₃COO(CH₂)₄OCOC₂₇H₅₃

L-a6: C₁₁H₂₁COO(CH₂)₄OCOC₁₇H₃₃

L-a7: C₁₇H₃₃COO(CH₂)₁₁OCOC₁₇H₃₃

L-a8: C₁₇H₃₃COOCH₂CH═CHCH₂OCOC₁₇H₃₃

L-a9: C₁₄H₂₇COOCH₂CH═CHCH₂OCOC₁₄H₂₇

L-a10: C₁₇H₃₃COO(CH₂)₈OCOC₁₄H₂₇

L-a11: C₁₇H₃₃COOCH₂C(CH₃)₂CH₂OCOC₁₇H₃₃

L-a12: C₁₀H₂₁COOCH₂C(CH₃)₂CH₂OCOC₁₀H₂₁

L-a13: C₁₃H₂₇COO(CH₂)₃OCOC₁₃H₂₇

Lubricant: Monoester

L-b1: C₁₇H₃₅COOC₁₇H₃₅

L-b2: C₁₇H₃₅COOC₄H₉

L-b3: C₁₇H₃₅COOCH₂CH₂OC₄H₉

L-b4: C₁₇H₃₅COO(CH₂CH₂O)₂C₄H₉

Support b-1: Polyethylene Terephthalate

Thickness: 62 μm

F-5 value:

MD: 114 MPa, TD: 107 MPa

Breaking strength:

MD: 276 MPa, TD: 281 MPa

Breaking extension:

MD: 174%, TD: 139%

Thermal shrinkage factor (80° C., 30 minutes):

MD: 0.04%, TD: 0.05%

Thermal shrinkage factor (100° C., 30 minutes):

MD: 0.2%, TD: 0.3%

Temperature expansion coefficient:

Long axis: 15×10⁻⁶/° C.

Short axis: 18×10⁻⁶/° C.

Central plane average surface roughness: 3 nm

Thickness unevenness: 1.5%

Support b-2: Polyethylene Naphthalate

Thickness: 55 μm

Central plane average surface roughness: 1.8 nm

Thermal shrinkage factor (80° C., 30 minutes):

MD: 0.007%, TD: 0.007%

Thermal shrinkage factor (100° C., 30 minutes):

MD: 0.02%, TD: 0.02%

Temperature expansion coefficient:

Long axis: 10×10⁻⁶/° C.

Short axis: 11×10⁻⁶/° C.

Thickness unevenness: 4.0%

Support b-3: Polyethylene Terephthalate

Thickness: 62 μm

Central plane average surface roughness: 9 nm

Thickness unevenness: 7.2%

Support b-4: Aramide

Thickness: 4.4 μm

Central plane average surface roughness: 2 nm

Orientation

o-1: Random orientation

o-2: Orientation in the machine direction using a Co magnet first, thenrandom orientation

o-3: Orientation in the machine direction using a Co magnet first, thenin the machine direction using a solenoid

o-4: Orientation in the vertical direction using a Co magnet

o-5: Orientation in the circumferential direction using a Co magnet

Backing Layer Coating Solution: bL-1 Fine carbon black powder 100 partsBP-800 (average particle size: 17 mμ, manufactured by Cabot Co., Ltd.)Coarse carbon black powder 10 parts Thermal Black (average particlesize: 270 mμ, manufactured by Cancarb Co., Ltd.) Calcium carbonate (softinorganic powder) 80 parts Hakuenka O (average particle size: 40 mμ,Mohs' hardness: 3, manufactured by Shiraishi Kogyo Co., Ltd.) α-Alumina(hard inorganic powder) 5 parts (average particle size: 200 mμ, Mohs'hardness: 9) Nitrocellulose resin 140 parts Polyurethane resin 15 partsPolyisocyanate 40 parts Polyester resin 5 parts Dispersant: Copperoleate 5 parts Copper phthalocyanine 5 parts Barium sulfate 5 partsMethyl ethyl ketone 2,200 parts Butyl acetate 300 parts Toluene 600parts

The above compositions of the coating solution for the backing layerwere blended in a continuous kneader, then dispersed with a sand mill.The resulting dispersion solution was filtered through a filter havingan average pore diameter of 1 μm to obtain a coating solution forforming the backing layer.

With respect to samples obtained by combining the above-described eachmethod arbitrarily as shown in Table 5 or 7, magnetic characteristics,central plane average surface roughness, areal recording density, etc.,were determined and the results obtained are shown in Table 6 or 8.

(1) Magnetic Characteristics (Hc):

Magnetic characteristics were measured using a vibrating samplemagnetometer (a product of Toei Kogyo Co., Ltd.) at Hm 10 KOe.

(2) Central Plane Average Surface Roughness (Ra):

Surface roughness (Ra) of the area of about 250 μm×250 μm, Rrms,peak-valley values were measured using “TOPO3D” (a product of WYKO Co.,Ltd., U.S.A.) by 3D-MIRAU method. The wavelength of measurement wasabout 650 nm and spherical compensation and cylindrical compensationwere applied. Measurement was performed using a light interference typenon-contact surface roughness meter.

(3) Linear Recording Density:

Linear recording density means a bit number of signals recorded per 1inch in the recording direction.

(4) Track Density:

Track density means a track number per 1 inch.

(5) Areal Recording Density:

Areal recording density means a value obtained by multiplying linearrecording density by track density.

(6) φm:

φm is the amount of magnetization per unit area of a magnetic recordingmedium, which is represented by Bm (Gauss) multiplying thickness. Thisis the value obtained by using a vibrating sample magnetometer (aproduct of Toei Kogyo Co., Ltd.) at Hm 10 KOe, which can be directlymeasured.

These linear recording density, track density and areal recordingdensity are values determined by systems to be used.

(7) Error Rate of Disc:

The above signals of linear recording density were recorded on the discby (2,7) RLL modulation system and error rate of the disc was measured.

(8) Error Rate of Tape:

The above signals of linear recording density were recorded on the tapeby 8-10 conversion PR1 equalization system and error rate of the tapewas measured using a DDS drive.

(9) Thickness of Magnetic Layer:

The sample having the thickness of about 0.1 μm was cut out with adiamond cutter in the machine direction of the magnetic medium, observedwith a transmission type electron microscope of from 10,000 to 100,000,preferably from 20,000 to 50,000 magnifications and photographed. Theprint size of the photograph was from A4 (i.e., 297×210 mm) to A5 (i.e.,210×148 mm) sizes. The present inventors paid attentions to thedifference of the shapes of the ferromagnetic powders and thenonmagnetic powders of the magnetic layer and the nonmagnetic layer andrimmed the interface of the magnetic layer and the nonmagnetic layer andalso the surface of the magnetic layer with black color by visualjudgement. Thereafter, the distance of the rimmed lines was measured bythe image processing apparatus “IBAS2” (manufactured by Zeiss Corp.).Measurement was conducted from 85 to 300 times when the length of thesample photograph was 21 cm. The average measured value at that time wastaken as d, and the standard deviation of the measured value was takenas σ. d depended on the description in JP-A-5-298653 and σ was obtainedby equation (2) in JP-A-5-298653. di means each measured value and n isfrom 85 to 300.

(10) I_(L)/I_(S):

The spatial frequency intensity in long wavelength of from 10 to 2 μm ofthe surface roughness of the magnetic layer (I_(L)) and the spatialfrequency intensity in short wavelength of from 1 to 0.5 μm of thesurface roughness of the magnetic layer (I_(S)) are intensities obtainedby conducting two dimensional Fourier transformation treatment to thesurface roughness profile data of the magnetic layer, then factorizingthe roughness components of every wavelength, and integrating theintensity within the range of the corresponding wavelength components.These are values calculated by the uptake of the range of 100 μm×100 μmas data of 512×512 pixels using an atomic force microscope (AFM)(manufactured by Digital Instruments, U.S.A.).

(11) Durability:

A floppy disc drive (“ZIP100”, a product of IOMEGA CORP., U.S.A.,rotation number: 2,968 rpm) was used. The head was fixed at the positionof radius of 38 mm. Recording was conducted at recording density of 34kbpi, then reproduced the signals recorded and this was taken as 100%.The disc was run under the following thermo-cycle condition, which beingtaken as one cycle, and those capable of running for 500 hours or morewere evaluated as grade o and those for less than 500 hours as grade x.Output was monitored every 24 hours of running and the point when theinitial reproduction output became 70% or less was taken as NG.

Thermo-Cycle Flow

25° C., 50% RH, 1 hr→(temperature up, 2 hr)→60° C., 20% RH, 7hr→(temperature down, 2 hr)→25° C., 50% RH, 1 hr→(temperature down, 2hr)→5° C., 50% RH, 7 hr→(temperature up, 2 hr)→(this cycle wasrepeated).

(12) Running Durability:

A floppy disc drive (“ZIP100”, a product of IOMEGA CORP., U.S.A.,rotation number: 2,968 rpm) was used. The head was fixed at the positionof radius of 38 mm. Recording was conducted at recording density of 34kfci, then reproduced the signals recorded and this was taken as 100%.The disc was run for 1,500 hours under the following thermocyclecondition, which being taken as one cycle. Output was monitored every 24hours of running and the point when the initial reproduction outputbecame 70% or less was taken as NG.

Thermo-Cycle Flow

25° C., 50% RH, 1 hr→(temperature up, 2 hr)→60° C., 20% RH, 7hr→(temperature down, 2 hr)→25° C., 50% RH, 1 hr→(temperature down, 2hr)→5° C., 50% RH, 7 hr→(temperature up, 2 hr)→(this cycle wasrepeated).

(13) Liner Wear:

A sample was run for 1,000 hours under the same condition as in theevaluation of running durability with the head being off, and aftercompletion of running, cartridge case of the sample was opened and thesurface of the magnetic layer was visually evaluated by the followingcriteria.

∘: No defect was observed on the surface of the magnetic layer.

Δ: Fine scratches were generated on a part of the surface of themagnetic layer.

x: Fine scratches were generated on the entire surface of the magneticlayer.

(14) Liner Adhesion:

A sample was run for 1,000 hours under the same condition as in theevaluation of running durability with the head being off, and aftercompletion of running, cartridge case of the sample was opened and thesurface of the magnetic layer was visually evaluated by the followingcriteria.

∘: Liner was not adhered on the surface of the magnetic layer.

Δ: Liner was adhered on a part of the surface of the magnetic layer.

x: Liner was adhered on the entire surface of the magnetic layer.

(15) Starting Torque:

Starting torque at the time of head-on in LS-102 drive (a product ofImation Co., Ltd.) was determined using torque gauge model 300 ATG (aproduct of Tonichi Seisakusho Co., Ltd.) (unit: g•cm).

(16) Measurement of C/Fe

C/Fe value was determined using Auger electron spectrometer PHI-660 typemanufactured by Φ Co. Conditions of measurement were as follows.

Accelerating voltage of primary electron beam: 3 KV

Electric current of sample: 130 nA

Magnification: 250-fold

Inclination angle: 30°

The C/Fe ratio is obtained as the C/Fe peak by integrating the valuesobtained by the above conditions in the region of kinetic energy of 130eV to 730 eV three times and finding the strengths of KLL peak of thecarbon and LMM peak of the iron as differentials.

TABLE 5 Disc Magnetic Layer Thickness Hc φm Lower Preparation Sample No.Prescription (μm) (Oe) I_(L)/I_(S) (emu/cm²) Layer Support MethodOrientation  1 mL-2 0.15 2,360 0.47 4.8 × 10⁻³ nU-1 b-1 Method 1 o-1  2mL-2 0.15 2,360 0.68 4.8 × 10⁻³ nU-2 b-1 Method 1 o-1  3 mL-2 0.15 2,3600.73 4.8 × 10⁻³ nU-4 b-1 Method 1 o-1  4 mL-2 0.15 2,360 0.96 4.8 × 10⁻³nU-5 b-1 Method 1 o-1  5 mL-2 0.05 2,400 1.03 1.6 × 10⁻³ nU-5 b-1 Method1 o-1  6 mL-2 0.10 2,380 1 3.2 × 10⁻³ nU-5 b-1 Method 1 o-1  7 mL-2 0.202,330 1.46 6.4 × 10⁻³ nU-5 b-1 Method 1 o-1  8 mL-2 0.15 2,360 1.38 4.8× 10⁻³ nU-5 b-2 Method 1 o-1  9 mL-1 0.15 2,550 1.33 4.2 × 10⁻³ nU-5 b-1Method 1 o-1 10 mL-4 0.15 2,500 0.79 2.1 × 10⁻³ nU-5 b-1 Method 1 o-1 11mL-5 0.15 2,500 0.88 2.4 × 10⁻³ nU-5 b-1 Method 1 o-1 12 mL-2 0.15 2,3600.52 4.8 × 10⁻³ nU-5 b-1 Method 3 o-1 13 mL-2 0.15 2,360 1.12 4.8 × 10⁻³nU-5 b-1 Method 1 o-2 14 mL-5 0.15 2,500 0.95 2.5 × 10⁻³ nU-5 b-1 Method1 o-2 15 mL-4 0.15 2,700 0.61 2.3 × 10⁻³ nU-5 b-1 Method 1 o-4 16 mL-20.15 2,660 1.25 4.8 × 10⁻³ nU-5 b-1 Method 5 o-5 17 mL-4 0.15 2,700 1.392.3 × 10⁻³ nU-5 b-1 Method 5 o-5 Comparative mL-2 0.15 2,360 1.86 4.8 ×10⁻³ nU-5 b-3 Method 1 o-1 Example 1 Comparative mL-3 0.15 1,600 1.334.8 × 10⁻³ nU-5 b-1 Method 5 o-4 Example 2 Comparative mL-3 0.15 1,6001.57 4.8 × 10⁻³ nU-5 b-1 Method 1 o-1 Example 3

TABLE 6 Linear Areal Track Recording Recording Error Density DensityDensity Rate Sample No. (tpi) (kbpi) (G bit/inch²) (10⁻⁵) Durability 15,200 144 0.75 0.2 ◯ 2 5,200 144 0.75 0.08 ◯ 3 5,200 144 0.75 0.03 ◯ 45,200 144 0.75 0.01 ◯ 5 5,200 144 0.75 0.06 ◯ 6 5,200 144 0.75 0.01 ◯ 75,200 144 0.75 0.2 ◯ 8 5.200 144 0.75 0.008 ◯ 9 5,200 144 0.75 0.004 ◯10 5,200 144 0.75 0.01 ◯ 11 5,200 144 0.75 0.005 ◯ 12 5,200 144 0.75 0.1◯ 13 5,200 144 0.75 0.001 ◯ 14 5,200 144 0.75 0.0006 ◯ 15 5,200 144 0.750.0004 ◯ 16 5,200 144 0.75 0.0002 ◯ 17 5,200 144 0.75 0.0001 ◯ 18 7,500200 1.5 0.8 ◯ 19 6,000 166 1.0 0.08 ◯ 20 3,000 120 0.36 0.007 ◯Comparative 5,200 144 0.75 10 ◯ Example 1 Comparative 5,200 144 0.75 40◯ Example 2 Comparative 5,200 144 0.75 29 x Example 3

TABLE 7 Computer Tape Magnetic Layer Sample Thickness Hc φm LowerPreparation Orienta- No. Prescription (μm) (Oe) I_(L)/I_(S) (emu/cm²)Layer Support Method tion 21 mL-2 0.15 2,460 0.48 4.8 × 10⁻³ nU-1 b-4Method 2 ◯-3 22 mL-2 0.15 2,460 0.69 4.8 × 10⁻³ nU-2 b-4 Method 2 ◯-3 24mL-2 0.15 2,460 0.97 4.8 × 10⁻³ nU-5 b-4 Method 2 ◯-3 25 mL-2 0.05 2,5001.04 1.6 × 10⁻³ nU-5 b-4 Method 2 ◯-3 26 mL-2 0.10 2,480 1.01 3.2 × 10⁻³nU-5 b-4 Method 2 ◯-3 27 mL-2 0.20 2,430 1.47 6.4 × 10⁻³ nU-5 b-4 Method2 ◯-3 28 mL-1 0.15 2,650 1.34 4.2 × 10⁻³ nU-5 b-4 Method 2 ◯-3 29 mL-20.15 2,460 0.53 4.8 × 10⁻³ nU-5 b-4 Method 4 ◯-3

TABLE 8 Linear Areal Track Recording Recording Error Sample DensityDensity Density Rate No. (tpi) (kbpi) (G bit/inch²) (10⁻⁵) Durability 213,000 122 0.366 0.09 ◯ 22 3,000 122 0.366 0.02 ◯ 23 3,000 122 0.3660.003 ◯ 24 3,000 122 0.366 0.001 ◯ 25 3,000 122 0.366 0.01 ◯ 26 3,000122 0.366 0.002 ◯ 27 3,000 122 0.366 0.01 ◯ 28 3,000 122 0.366 0.0005 ◯29 3,000 122 0.366 0.02 ◯ 30 4,000 150 0.6 0.02 ◯ 31 5,000 170 0.85 0.5◯

As described above, the above signals of linear recording density wererecorded on the tape by 8-10 conversion PR1 equalization system anderror rate of the tape was measured using a DDS drive. In each of SampleNos. 30 and 31, the tape of Sample No. 24 was used and error rate wasmeasured with varying linear recording density and track density.

From the results in Tables 5 and 7, it can be seen that the error ratesof the magnetic recording media according to the present inventionhaving I_(L)/I_(S) of less than 1.5, in particular, the error rates inhigh density recording region, are 1×10⁻⁵ or less, which areconspicuously excellent as compared with conventional disc-like mediahaving I_(L)/I_(S) of 1.5 or more.

TABLE 9-1 Disc Lubricant Lubricant for Upper Magnetic Layer for LowerNonmagnetic Layer Medium Sample Amount Amount Amount Amount No. No. Kind(part) Kind (part) Kind (part) Kind (part) 1 1 L-a3 6 L-b2 6 L-a3 8 L-b28 2 1 L-a3 6 L-b3 6 L-a3 8 L-b3 8 3 1 L-a3 6 L-b4 6 L-a3 8 L-b4 8 4 1L-a3 9 L-b3 3 L-a3 12 L-b3 4 5 1 L-a3 3 L-b3 9 L-a3 4 L-b3 12 6 1 L-a3 6L-b3 6 L-a3 12 L-b3 4 7 1 L-al 6 L-b3 6 L-al 8 L-b3 8 8 1 L-a8 6 L-b3 6L-a8 8 L-b3 5 9 1 L-a3 6 L-b3 6 L-al 8 L-b3 8 10 1 L-a3 6 L-b3 6 L-a3 8L-b2 8 11 1 L-a3 6 L-b2 6 L-a3 8 L-a3 8 12 1 L-a3 6 L-b3 3 L-a3 8 L-b3 813 1 L-a3 6 L-b3 3 L-a3 8 L-b3 5 14 1 L-a3 6 L-b3 6 L-a3 8 L-b3 5 15 1L-a3 3 L-b3 3 L-a3 5 L-b3 5 16 1 L-a3 8 L-b3 8 L-a3 10 L-b3 10 17 1 L-a36 L-b3 6 L-a3 10 L-b3 8 18 3 L-a3 6 L-b3 6 L-a3 8 L-b3 8 19 6 L-a3 6L-b3 6 L-a3 8 L-b3 8 20 9 L-a3 6 L-b3 6 L-a3 8 L-b3 8

TABLE 9-2 Disc Medium Sample Running Liner Liner Starting No. No. C/FeDurability Wear Adhesion Torque  1 1 55 1,200 ◯ ◯ 60  2 1 50 1,500 ◯ ◯53  3 1 50 1,440 ◯ ◯ 59  4 1 35 1,500 ◯ ◯ 55  5 1 70 1,080 ◯ ◯ 65  6 140 1,500 ◯ ◯ 56  7 1 25 1,008 ◯ ◯ 64  8 1 20 1,200 ◯ ◯ 54  9 1 30 1,104◯ ◯ 63 10 1 50 1,032 ◯ ◯ 65 11 1 55 1,008 ◯ ◯ 60 12 1 45 1,500 ◯ ◯ 54 131 40 1,464 ◯ ◯ 54 14 1 40 1,440 ◯ ◯ 55 15 1 30 1,008 ◯ ◯ 50 16 1 651,320 ◯ ◯ 68 17 1 60 1,500 ◯ ◯ 55 18 3 95 1,008 ◯ ◯ 65 19 6 90   972 ◯ ◯70 20 9 90 1,080 ◯ ◯ 61

TABLE 10-1 Disc Lubricant for Upper Magnetic Layer Lubricant for LowerNonmagnetic Layer Medium Sample Amount Amount Amount Amount No. No. Kind(part) Kind (part) Kind (part) Kind (part) 21 10  L-a3  6 L-b3 6 L-a3 10L-b3 8 22 11  L-a3  6 L-b3 6 L-a3 10 L-b3 8 23 15  L-a3  6 L-b3 6 L-a310 L-b3 8 24 1 L-a3  4 L-b2 4 L-a3  6 L-b2 6 L-b3 4 L-b3 6 25 1 L-a1  4L-b2 4 L-a3  6 L-b2 6 L-b4 4 L-b4 6 26 1 L-a1 12 L-a1 16 27 1 L-a3 12L-a3 16 28 1 L-a8 12 L-a8 16 29 1 L-b2 12 L-b2 16 30 1 L-a3 12 L-b3 1631 1 L-b3 12 L-a3 16 32 1 L-a1  6 L-a3 6 L-a1  8 L-a3 8 33 1 L-b2  6L-b3 6 L-b2  8 L-b3 8 34 2  L-a11  8 L-b3 2  L-a11  8 L-b3 2 35 2  L-a1112 L-b3 2  L-a11 12 L-b3 2 36 2  L-a11 20 L-b3 2  L-a11 20 L-b3 2 37 2L-a3 14 L-b3 2 L-a3 14 L-b3 2 38 2  L-a12 16 L-b3 2  L-a12 16 L-b3 2 392  L-a13 16 L-b3 2  L-a13 16 L-b3 2

TABLE 10-2 Disc Medium Sample Running Liner Liner Starting No. No. C/FeDurability Wear Adhesion Torque 21 10  10 1,500 ◯ ◯ 59 22 11  15 1,224 ◯◯ 62 23 15  10 1,320 ◯ ◯ 65 24 1 40 1,500 ◯ ◯ 56 25 1 35 1,416 ◯ ◯ 55 261 10   552 ◯ ◯ 52 27 1 15   600 ◯ ◯ 50 28 1 10   576 Δ ◯ 51 29 1 85  480 ◯ Δ 80 30 1 70   528 ◯ Δ 73 31 1 20   600 ◯ ◯ 64 32 1 10   576 ◯ ◯59 33 1 80   480 ◯ Δ 81 34 2  5 1,500 ◯ ◯ 30 35 2 18 1,500 ◯ ◯ 35 36 280 1,500 ◯ ◯ 45 37 2 36 1,500 ◯ ◯ 40 38 2 48 1,500 ◯ ◯ 40 39 2 45 1,500◯ ◯ 40

As is apparent from the results in Tables 9 and 10, when monoester anddiester lubricants according to the present invention are used incombination, running durability, liner wear, liner adhesion and startingtorque were markedly improved.

Further, as the results of experiment are not described, when the abovemonoester and diester lubricants are used in computer tapes incombination, the magnetic medium of the present invention exhibitedexcellent characteristics, e.g., low friction coefficient, excellentdurability without causing clogging, and excellent abrasion resistanceafter smooth 100 passes or even after 1,000 passes.

EFFECT OF THE INVENTION

The present invention can be attained by a magnetic recording mediumwhich comprises a support having thereon a substantially nonmagneticlower layer and a magnetic layer comprising a ferromagnetic metal powderor a ferromagnetic hexagonal ferrite powder dispersed in a binderprovided on the lower layer, which is a magnetic recording medium forrecording signals of from 0.17 to 2 G bit/inch² of areal recordingdensity, wherein the coercive force of the magnetic layer is 1,800 Oe ormore, and the thickness unevenness of the support is 5% or less of thethickness of the support, or the present invention can be attained by amagnetic recording medium comprising the above constitution, wherein theratio of the spatial frequency intensity in long wavelength of from 10to 2 μm of the surface roughness of the magnetic layer I_(L) to thespatial frequency intensity in short wavelength of from 1 to 0.5 μm ofthe surface roughness of the magnetic layer I_(S), I_(L)/I_(S), is lessthan 1.5. The magnetic layer preferably has a dry thickness of from 0.05to 0.30 μm, Φm of preferably from 10.0×10⁻³ to 1.0×10⁻³ emu/cm², and themagnetic recording medium of the present invention is a magneticrecording medium for recording signals of preferably from 0.20 to 2 Gbit/inch² of areal recording density. The magnetic recording mediumhaving high capacity, excellent high density characteristics andexcellent durability, in which, in particular, the error rate in highdensity recording region has been markedly improved, which could neverbe obtained by conventional techniques, could be obtained by adoptingthe constitution of the present invention.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A magnetic recording medium which comprises a support having thereon a substantially nonmagnetic lower layer and a magnetic layer provided on said lower layer, said magnetic layer comprising a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder, a lubricant, and an abrasive dispersed in a binder said medium capable of recording data at an areal recording density of from 0.17 to 2 G bit/inch² said magnetic layer having a coercive force of 1,800 Oe or more, and said support having a thickness unevenness of 5% or less of the thickness of said support and the ratio of the spatial frequency intensity in long wavelength of from 10 to 2 μm of the surface roughness of said magnetic layer (I_(L)) to the spatial frequency intensity in short wavelength of from 1 to 0.5 μm of the surface roughness of said magnetic layer (I_(S)), (I_(L)/I_(S)), is less than 1.5.
 2. The magnetic recording medium as claimed in claim 1, wherein the thickness unevenness of said support is 2% or less of the thickness of said support.
 3. The magnetic recording medium as claimed in claim 1, wherein the ratio of the spatial frequency intensity in long wavelength of from 10 to 2 μm of the surface roughness of the magnetic layer I_(L) to the spatial frequency intensity in short wavelength of from 1 to 0.5 μm of the surface roughness of the magnetic layer I_(S), I_(L)/I_(S), is 0.5≦I_(L)/I_(S)≦1.3.
 4. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer has a dry thickness of from 0.05 to 0.30 μm, and φm of from 10.0×10⁻³ to 1.0×10⁻³ emu/cm².
 5. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer has a dry thickness of from 0.05 to 0.25 μm, and φm of from 8.0×10⁻³ to 1.0×10⁻³ emu/cm².
 6. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for recording signals of from 0.20 to 2 G bit/inch² of areal recording density.
 7. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer has a central plane average surface roughness of 4.0 nm or less measured by 3D-MIRAU method.
 8. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer has a coercive force of 2,100 Oe or more, and said ferromagnetic metal powder has an average long axis length of 0.12 μm or less or said ferromagnetic hexagonal ferrite powder has an average particle size of 0.10 μm or less.
 9. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for a system of a high transfer rate of 1.0 MB/sec. or more.
 10. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for a high capacity floppy disc system of disc rotation speed of 2,000 rpm or more.
 11. The magnetic recording medium as claimed in claim 1, wherein said ferromagnetic metal powder comprises Fe as a main component, has an average long axis length of from 0.12 μm or less, and an acicular ratio of from 4.0 to 9.0.
 12. The magnetic recording medium as claimed in claim 1, wherein said ferromagnetic metal powder comprises Fe as a main component, has an average long axis length of 0.10 μm or less, and a crystallite size of from 80 to 180 Å.
 13. The magnetic recording medium as claimed in claim 1, wherein said support has a central plane average surface roughness of 4.0 nm or less measured by 3D-MIRAU method.
 14. The magnetic recording medium as claimed in claim 1, wherein said support has a thermal shrinkage factor of 0.5% or less both at 100° C. for 30 minutes and at 80° C. for 30 minutes in every direction of in-plane of said support.
 15. The magnetic recording medium as claimed in claim 1, wherein said support has a temperature expansion coefficient of from 10⁻⁴ to 10⁻⁸/° C. in every direction of in-plane of said support.
 16. The magnetic recording medium as claimed in claim 1, wherein said lower layer and/or magnetic layer contain(s) at least three fatty acids, or three fatty acid esters, or a combination of three materials selected from a group of materials consisting of fatty acids and fatty acid esters.
 17. The magnetic recording medium as claimed in claim 16, wherein said fatty acid and said fatty acid ester have the same fatty acid residues with each other.
 18. The magnetic recording medium as claimed in claim 17, wherein said fatty acid contains at least a saturated fatty acid and said fatty acid ester contains at least a saturated fatty acid ester or an unsaturated fatty acid ester.
 19. The magnetic recording medium as claimed in claim 16, wherein said fatty acid ester contains a monoester and a diester.
 20. The magnetic recording medium as claimed in claim 16, wherein said fatty acid ester contains a saturated fatty acid ester and an unsaturated fatty acid ester.
 21. The magnetic recording medium as claimed in claim 1, wherein the surface of said magnetic layer has a C/Fe peak ratio of from 5 to 120 when the surface is measured by the Auger electron spectroscopy.
 22. The magnetic recording medium as claimed in claim 1, wherein said lower layer contains a carbon black having a particle size of from 5 to 80 mμ and said magnetic layer contains a carbon black having a particle size of from 5 to 300 mμ.
 23. The magnetic recording medium as claimed in claim 1, wherein said lower layer contains a carbon black having an average particle size of from 5 to 80 mμ and a carbon black having an average particle size of larger than 80 mμ.
 24. The magnetic recording medium as claimed in claim 1, wherein said lower layer and said magnetic layer each contains a carbon black having an average particle size of from 5 to 80 mμ.
 25. The magnetic recording medium as claimed in claim 1, wherein said lower layer contains an acicular inorganic powder having an average long axis length of 0.20 μm or less and an acicular ratio of from 4.0 or 9.0.
 26. The magnetic recording medium as claimed in claim 1, wherein said lower layer contains an acicular inorganic powder and said magnetic layer contains an acicular ferromagnetic metal powder, and the average long axis length of said acicular inorganic powder is from 1.1 to 3.0 times the average long axis length of said acicular ferromagnetic metal powder.
 27. The magnetic recording medium as claimed in claim 1, wherein said lower layer and/or magnetic layer contain(s) a phosphorus compound and said lower layer contains an acicular or spherical inorganic powder.
 28. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer contains at least an abrasive having an average particle size of from 0.01 to 0.30 μm.
 29. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer contains at least a diamond particle having an average particle size of from 0.01 to 1.0 μm.
 30. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer contains two kinds of abrasives having a Mohs' hardness of 9 or more.
 31. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer contains an α-alumina and a diamond particle.
 32. The magnetic recording medium as claimed in claim 1, wherein said lower layer and/or magnetic layer contain(s) at least a polyurethane having a glass transition temperature of from 0° C. to 100° C.
 33. The magnetic recording medium as claimed in claim 1, wherein said lower layer and/or magnetic layer contain(s) at least a polyurethane having a breaking stress of from 0.05 to 10 kg/mm².
 34. The magnetic recording medium as claimed in claim 1, wherein said magnetic layer has a dry thickness of from 0.05 to 0.20 μm and said magnetic layer contains an abrasive having an average particle size of 0.4 μm or less.
 35. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a disk magnetic recording medium.
 36. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for a high capacity floppy disc system of disc rotation speed of 3,000 rpm or more.
 37. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for a system of a high transfer rate of 2.0 MB/sec. or more.
 38. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium which has realized subordination transposition capable of recording/reproduction with conventional 3.5 inch type floppy discs.
 39. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for a high capacity floppy disc system adopting a dual discrete gap head having both a narrow gap for high density recording and a broad gap for conventional 3.5 inch type floppy discs.
 40. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for a high capacity floppy disc system adopting a head which floats by disc rotation.
 41. The magnetic recording medium as claimed in claim 1, wherein said magnetic recording medium is a magnetic recording medium for a high capacity floppy disc system adopting a head which floats by disc rotation and, at the same time, a linear type voice coil motor as a driving motor of the head.
 42. A magnetic recording medium which comprises a support having thereon a substantially nonmagnetic lower layer and a magnetic layer provided on said lower layer, said magnetic layer comprising a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder, a lubricant, and an abrasive dispersed in a binder said medium capable of recording data at an areal recording density of from 0.17 to 2 G bit/inch² said magnetic layer having a coercive force of 1,800 Oe or more, and the ratio of the spatial frequency intensity in long wavelength of from 10 to 2 μm of the surface roughness of said magnetic layer (I_(L)) to the spatial frequency intensity in short wavelength of from 1 to 0.5 μm of the surface roughness of said magnetic layer (I_(S)), (I_(L)/I_(S)), is less than 1.5.
 43. The magnetic recording medium of claim 1, wherein said support is a flexible polymeric material less than about 62μ thick.
 44. A magnetic recording medium which comprises a support having thereon a substantially nonmagnetic lower layer and a magnetic layer provided on said lower layer, said magnetic layer comprising a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder, a lubricant, and an abrasive dispersed in a binder, said magnetic layer having a coercive force of 1,800 Oe or more, and said support having a thickness unevenness of 5% or less of the thickness of said support and the ratio of the spatial frequency intensity in long wavelength of from 10 to 2 μm of the surface roughness of said magnetic layer (I_(L)) to the spatial frequency intensity in short wavelength of from 1 to 0.5 μm of the surface roughness of said magnetic layer (I_(S)), (I_(L)/I_(S)), is less than 1.5.
 45. The magnetic recording medium of claim 44, wherein said medium is in the condition of having information recorded thereon in an areal recording density range of from 0.17 to 2 G bit/inch².
 46. The magnetic recording medium of claim 44, wherein said support is a flexible polymeric material less than about 62μ thick. 